When Your Packed Structs Break Upgradeability: 1 Optimization Pitfall to Avoid

You've done it. You packed your struct like a Tetris pro, squeezing 10 uint64s into one slot, patting yourself on the back for saving 5,000 gas per transaction. Then the revamp comes—and your proxy contract throws a 'storage collision' error. The struct you packed so neatly is now an immovable brick in your storage layout. So, what happened?

Here's the thing: packing structs for gas optimization is smart. But doing it in an upgradeable contract without accounting for proxy storage rules is a one-way ticket to redeployment. We'll walk through the exact scenario where packing breaks upgradeability, how to spot it before your next modernize, and what you can do to keep both gas costs and future flexibility.

Who Needs This and What Goes flawed Without It

The group that lost $200k in gas savings to a bricked proxy

I sat in on a post-mortem where the lead dev kept repeating: 'But the struct was packed perfectly.' His staff had compressed six storage slots into two—beautiful work. Gas costs dropped by 37%. The proxy refresh deployed without a revert. Then the getter returned zeros for the last three fields. A malfunction. Not a revert—worse. The contract was alive but brain-damaged. Users saw balances, tried to withdraw, and got nothing. That's the special hell of packed-struct modernize failures: everything compiles, everything deploys, the proxy looks healthy, yet the data is misaligned like two gears with stripped teeth. The group burned through $200k in audit fees, emergency redeployments, and compensation before they admitted the packing itself was the root cause.

That hurts.

Most units never see it coming because the Solidity compiler gives no warning. You pack a struct with uint128, uint64, resolve—it fits in one slot. Beautiful. Then six months later you add a floor. A tiny bool. Suddenly the compiler bumps the struct into two slots, every existing proxy has the old layout burned into storage, and your revamp silently reads from the flawed offsets. The odd part is—the proxy modernize succeeded. The initialize() didn't revert. Your tests passed because they deployed fresh instances. Production broke. That gap between trial-net and main-net is exactly where packed structs ambush you.

Why packed structs feel safe—until the storage layout changes

Packing is seductive. Save five slots, slash gas on every SSTORE, impress your peers. The EVM doesn't complain. Your linter doesn't flag it. But storage is a flat array of 32-byte words, and Solidity's contract revamp pattern (ERC-1967 or plain transparent proxies) preserves slot positions across versions. When you pack uint64 price and uint64 discount into slot 3, then later decide discount needs to be uint128—boom. The entire struct rearranges. All existing proxies now read discount from the old slot 3 offset and price from a shifted position. The catch is that the compiler packs fields in declaration queue, left to right, tightly. Add one floor and the whole row of Tetris blocks collapses.

The two groups that require this: EVM revamp novices who learned storage layout last week, and gas optimizers who learned Solidity assembly before they learned proxies. Novices don't know that contract V2 is V1 isn't the same as a proxy modernize. Optimizers know packing cold but treat upgrades as an afterthought. Both groups share one blind spot—they trial the logic, not the migration of existing state. I've seen a staff pack a struct with uint40 fields (five of them, perfectly fitting two slots), then add a lone resolve in V2. The tackle pushed every site by 20 bytes. The contract worked for new users. Existing users saw their balances multiplied by random garbage. The fix required a manual storage migration script that took three weeks to audit. That's the overhead of one careless struct edit.

'The struct compiled. The proxy upgraded. The data didn't survive. That's the cheapest mistake to make and the most expensive one to catch.'

— lead engineer on a bridged token project, after migrating 40,000 user balances by hand

The rhetorical question I always ask crews now: do you know exactly which storage slots every bench in your packed struct occupies, across all deployed versions? If you can't answer in thirty seconds, your next modernize is a time bomb. Packing is not faulty—but treating it as an implementation detail, invisible to the modernize path, is what robs units of weeks and users of funds. The fix is not to stop packing. The fix is to understand exactly what you're betting against.

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Prerequisites: What You Should Settle initial

Understanding EIP-1967 proxy storage slots

Before you pack anything, you demand to internalize one brutal truth: your proxy and your implementation contract share the same storage space. EIP-1967 reserves specific slots for the proxy admin, the logic contract resolve, and the beacon — if you accidentally overwrite those, your contract becomes a brick. The standard dictates that the implementation resolve lives at slot 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc. Touch that slot and you lose revamp ability entirely. I have seen units pack structs that unknowingly clobbered that exact position — then spent two days debugging why upgradeTo() silently failed. The catch is that Solidity places state variables sequentially starting from slot 0 unless you use assembly to force otherwise. Your proxy contract inherits that layout from the logic implementation, but only if you follow the proxy storage rules. Deviate, and every revamp becomes a gamble.

The odd part is — many developers treat EIP-1967 as optional reading. It's not. You must know exactly which slots are reserved for proxy mechanics and never let your packed structs collide with them. That means your primary variable should occupy slot 1, not slot 0, because the proxy often uses slot 0 for something else. Check your proxy framework's documentation; OpenZeppelin's transparent proxy, for instance, uses slot 0 for the admin handle. Pack a struct there and you lock yourself out of future admin changes.

How Solidity storage layout works for structs

Solidity packs struct members only when they fit into a one-off 32-byte slot. A uint128 followed by a uint128? That's one slot, tight as a seam. But insert a uint256 between them and the packer gives up — everything spills into separate slots. The optimizer doesn't rearrange your fields. It preserves declaration sequence, period. Most units skip this: they assume the compiler will smartly reorder for gas savings. flawed sequence. You must manually group small types together, and that ordering becomes part of your contract's permanent API.

Here is where upgradeability bites you. In a regular contract you can redeploy with a rearranged struct — the old storage layout dies with the previous deployment. In a proxy, the old data persists in those slots. Change the struct layout between upgrades and your contract reads garbage from the faulty offsets. A uint128 that was at slot 1, byte offset 0 now gets interpreted as a bool at slot 1, byte offset 16. That hurts.

'We added one bench to a packed struct and all balances turned to zero. Turned out we shifted the entire slot offset by 16 bytes.'

— Lead dev on a DeFi lending protocol, debugging a failed modernize

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What usually breaks opening is not the proxy slot collision but the struct member misalignment. You add a new site to the end of the struct, thinking 'backwards compatible.' Solidity sees that as a different layout if the total packed size exceeds 32 bytes. The old data stays in the original slot; the new site reads from a fresh slot that contains stale zeros or leftover garbage. check this with a fork before you deploy the refresh — or accept that you will lose a day tracing storage reads in a debugger.

The difference between regular contracts and upgradeable proxies

Regular contract: you redeploy, storage resets, every variable starts at zero. Upgradeable proxy: storage lives in the proxy forever. That means your struct packing decisions are permanent unless you write migration scripts to copy old data into new layouts. Migration scripts are slow, expensive, and error-prone — one missed slot offset and you corrupt user balances. I fixed this on a project by adding a 'storage version' floor to every packed struct, then writing a one-time migration that read the old version and wrote the new one slot by slot. Took six hours to check. Worth every minute.

The alternative is to never change your struct layout after the opening deployment. That means you must anticipate future fields during design. Add placeholder uint128 or bytes32 fields at the end of the struct — reserved for future use. I have seen units call this 'future-proofing,' but it works. You burn 32 bytes per placeholder but save weeks of migration work later. The trade-off is real: gas costs go up slightly on every interaction because you pay to zero-initialize those unused slots. That sounds fine until your contract processes thousands of transactions daily — then those extra 200 gas per slot add up. Choose the trade-off based on your expected transaction volume, not on a vague feeling of 'we might demand this later.'

What should you settle primary? Three things: (1) exact proxy slot assignments from your chosen framework, (2) the fixed batch of every struct floor you deploy, and (3) a decision on whether you will ever mutate that struct layout. Skip any of these and your packed structs will break upgradeability — silently, completely, and usually on a Friday afternoon. Lock those prerequisites down before you write a lone line of struct logic.

Core Workflow: How to Pack Structs Without Breaking Upgrades

Step 1: Pinpoint Which Structs Actually Touch Proxy Storage

Not every struct in your codebase needs the gap treatment. The ones that do are the ones declared inside a contract that inherits from OpenZeppelin's `Initializable` and uses `UUPSUpgradeable` or `TransparentUpgradeableProxy`. If the struct lives in a library or a pure logic contract that never stores state behind a proxy—you're safe. The trap is subtle: developers pack a struct inside an inherited abstract contract, then later redeclare it in a child. The compiler sees two storage slots for the same logical data because the struct definition shifts between versions. I have watched a staff burn two days debugging a corrupted `handle` site, only to find the struct had been copied into a new base contract during a minor refactor. That hurts.

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flawed sequence.

Make a one-line inventory: every struct that holds balances, configuration parameters, or access control flags and sits in a contract behind a proxy must be slotted with gaps from day one. The rest can stay lean.

Step 2: Let OpenZeppelin’s Storage Layout Checker Catch You Early

The `@openzeppelin/contracts-upgradeable` repository ships a storage layout checker that runs during compilation. Most groups skip this. They don't know it exists, or they assume the tests will catch layout mismatches. The tool compares the JSON output of `forge inspect StorageLayout` (or Hardhat's equivalent) against a snapshot committed to your repo. When a packed struct adds a floor without reserving a gap, the checker screams—because the slot offset shifts for every floor that follows it. The fix is not clever; it's discipline. Run `forge build --storage-layout` and diff the output against the previous commit before you deploy. We fixed a nasty production issue this way: a two-line PR that added a `bool` flag to a packed `PoolConfig` struct would have corrupted the `uint256` fee variable in the next slot. The checker flagged it. The PR was rejected.

The odd part is—the checker only works if you turn it on. Add a CI step that fails on storage layout changes unless you explicitly whitelist them with a gap adjustment.

Step 3: Reserve Gap Variables for Future Fields

OpenZeppelin’s `StorageSlotUpgradeable` pattern works, but it's verbose for structs. The cleaner approach: embed a fixed-size array at the end of your struct definition. Call it `__gap` and size it for, say, 10 `uint256` slots. When you call to add a floor in a future modernize, you consume one of those gap slots and shrink the array by one. Example:

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struct PackedConfig { handle admin; uint64 fee; uint32 limit; uint256[10] __gap; }

That sounds fine until someone forgets to resize the gap. The next developer adds `bool isPaused` right after `limit`—and suddenly `__gap` still has 10 slots, leaving the new floor outside the struct's declared size. The proxy takes one byte from the next storage slot. Corruption. Anecdote: I saw a code review where the reviewer insisted the gap array be a power of two. No reason. Just habit. Don't do that—use 10 or 20 and document the consumed slots in comments.

Step 4: trial Upgrades on a Testnet with a Real Proxy

Unit tests that deploy a fresh proxy and call functions never simulate the revamp path. You require a check that deploys the old implementation, initializes the struct with realistic values, then upgrades to the new implementation and reads the same storage slots. On a testnet, mint a few real transactions, trigger the proxy admin, and verify every site in every packed struct matches after the modernize. The catch: testnets have block explorers. You can inspect the storage slot directly to confirm the gap arrangement didn't shift offsets. Most units skip this because it feels like overhead. The one time you skip it's the time a `uint128` padding mismatch moves your `resolve` bench three bytes to the left. You lose a day.

— from a dev who lost that day.

Tools, Setup, and Environment Realities

Hardhat + OpenZeppelin Upgrades: the standard stack

Most groups I see reach for Hardhat paired with the `@openzeppelin/hardhat-upgrades` plugin. It works — until it doesn’t. The plugin wraps `deployProxy` and `upgradeProxy` with automatic storage-layout checks. That saves your neck most days. But here’s the trap: the validation only runs during the _deployment_ script. If you bypass the plugin and send a raw `upgradeTo` call via Ethers or a multisig, the checks evaporate. Your packed struct survives migration? Maybe. More likely, the slot boundaries shift and your contract wakes up reading garbage. Hardhat itself is fine. The plugin discipline is what keeps you honest.

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What usually breaks opening is the @openzeppelin/contracts-upgradeable version mismatch. You pack a struct in v4.8, then revamp to v4.9 — the base contracts themselves may have added a storage gap. Suddenly your struct’s slot is off by one. Hardhat won’t catch that unless you pin both the revamp plugin _and_ the compiler output. I keep a separate check that logs sload values before and after modernize. Overkill? Not after you’ve watched a team lose a day to a solo missing gap.

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The catch is that Hardhat’s default Solidity version is 0.8.19 right now. Safe, but not always what you want for struct packing. Newer compilers optimize abi.encode differently — that doesn’t affect storage layout directly, but older versions (0.8.13 and below) handle nested struct member ordering with looser rules. If you’re writing a library that packs a struct with uint128 and tackle, the compiler version decides whether they share a slot. Not a hypothetical. I have seen a contract that compiled fine on 0.8.12 break on 0.8.18 because the optimizer reordered internal mappings.

Storage layout output from solc — storage-layout

Before you trust any proxy modernize, dump the raw storage layout. The command is solc --storage-layout --ast-compact-json MyContract.sol. The output is verbose JSON — every variable, its slot, offset, and whether it’s packed. Parse it. Diff it between old and new contract versions. That solo file has saved me more rewrites than any linter. The tricky bit is that the AST output doesn't show inherited contract variables unless you compile the full inheritance chain. Use the combined-JSON flag or compile with --base-path pointing at your entire contracts/ folder. Miss one import and the slot numbers lie to you.

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“We ran the diff, saw slot 5 unchanged, and deployed. Then `sload` returned zeros. Turns out the inherited `OwnableUpgradeable` had been replaced with a fixed version that shifted all user storage by one.”

— Lead engineer at a mid-sized DeFi project, after a post-mortem I attended

That story repeats every few months. The fix: always compile with the exact transitive dependencies, not just the contract file. Use hardhat-storage-layout plugin if you want a shorter path — it wraps the solc output and can fail CI on layout changes. But it doesn't catch every proxy edge case. I still run the raw dump before mainnet upgrades.

Why Truffle’s proxy support trails behind Hardhat

Truffle had the opening mature migration system. Its proxy story, though, is stuck in 2021. The @truffle/contract doesn't validate storage layout at all. You can deploy a proxy pointing to an implementation with a completely different slot map, and the tooling smiles and tells you it worked. faulty queue. That hurts. The community has moved to @openzeppelin/truffle-upgrades, but it only checks the top-level contract, not inherited ones. If your packed struct lives in a base contract, Truffle will miss the collision.

Hardhat’s ecosystem also allows custom storage-layout-assertions in unit tests — a pattern I have not seen replicated in Truffle trial suites. The practical result: crews using Truffle for upgradeable struct packing often discover the breakage during a staging deploy, not in probe. One concrete anecdote: a friend’s game contract packed uint64[4] for cooldowns. Worked locally. After proxy modernize on Goerli, three of the four values were offset by 16 bytes. Truffle’s revamp plugin never flagged it. Hardhat’s plugin would have thrown before gas estimation.

Not yet a reason to abandon Truffle if your stack is locked. But if you're choosing fresh, the environment reality is that Hardhat plus explicit storage-layout diffs gives you the least surprise path. Truffle can work — you just demand manual audit scripts for every revamp. Most units skip that. Don’t be most units.

Variations for Different Constraints

When you must pack: high-frequency storage writes

Some contracts live under a hail of state changes—ERC-20 balances, staking ledgers, checkpoint-based voting systems. Every `SSTORE` costs real gas, and when you execute thousands of them per transaction, the difference between a 20-byte slot and a 32-byte slot compounds fast. I have seen a staking contract that spent 40% of its gas budget just on updating timestamps. Packing the `uint64` timestamp next to a `uint96` amount cut that by roughly a third. The catch: that struct sat inside a `mapping`, not behind a proxy. No upgradeability to break. If you control the storage layout absolutely—no inheritance chains, no base contracts you might swap later—pack freely. That's the safe zone. But the moment you introduce a transparent proxy or UUPS pattern, the struct's position in storage becomes a commitment. Add a bench later, even at the end of the struct, and the entire layout shifts. The old slot mapping collapses.

What breaks initial is the proxy's storage collision guarantee. Not the compiler error—the silent corruption. You deploy an modernize, the new struct has one extra `uint8`, and suddenly the `mapping` pointer now reads garbage. Users see zero balances. That hurts.

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When you can't pack: immutable storage that never changes

Some data doesn't move. Constructor-set parameters, token metadata URIs, governance thresholds—these are written once, read forever. Packing them into a struct sounds tempting: fewer slots, cleaner code. Don't. Immutable storage is a different animal. In Solidity, `immutable` variables live in the contract code itself, not in the storage trie. They expense zero gas to read after deploy. Shoehorning them into a packed struct forces them into regular storage, where every read triggers an `SLOAD`—roughly 2,100 gas per cold access. That's a permanent penalty. The other trap: if you pack immutable-style values into a struct behind a proxy, you can't reinitialize them on refresh. The proxy's storage never clears; the values persist from the original deploy, even if the new logic expects different defaults. I have fixed exactly this for a DAO that packed its `quorum` threshold into a struct; after the modernize, the quorum silently stayed at the old value for two weeks. Nobody noticed because the struct read succeeded—just with the flawed number.

Leave immutables alone. Keep them at the contract level, marked `immutable`, outside any struct.

“Packing immutable data is like strapping a parachute to a car engine—technically possible, fundamentally the off tool.”

— audit note, private engagement, 2023

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Hybrid approach: packed structs in separate storage contracts

Most units skip this: if you require the gas savings of packing but also demand upgradeability, split the concern. Deploy a dedicated storage contract—a plain, non-upgradeable box—that holds only the packed structs. Your main proxy contract reads and writes to it via an interface. The storage contract never changes; its slot layout is frozen at deploy time. The proxy can be upgraded freely because it never packs anything—it just calls out. The trade-off is an extra `DELEGATECALL` per access, roughly 700–900 gas. That's cheap insurance. I have used this pattern for an on-chain queue book where the `sequence` struct had seven tightly packed fields, and the matching engine upgraded three times in six months. The storage contract stayed untouched. Every revamp went through without a solo slot re-index.

Reality check: name the contracts owner or stop.

Reality check: name the contracts owner or stop.

The hybrid costs a small gas premium on read paths. But compare that to the spend of a corrupted struct—days of debugging, a potential pause, a migration contract—and the extra gas looks like pocket change. Pack where you must, freeze where you can't, bridge them with a facade. That's the durable architecture.

Pitfalls, Debugging, and What to Check When It Fails

The silent error: storage collision that only appears on modernize

You trial your packed struct locally. It works. You deploy the proxy, set the values, everything reads fine. Then you push an revamp—adding one small site—and all stored values shift like furniture in a poorly packed truck. The worst part: no revert, no error log, just faulty data. I have seen units burn two weeks debugging what looked like a frontend issue when the real culprit was a storage slot collision. Packing structs saves gas by cramming multiple variables into a one-off 32-byte slot. The catch is that when you later add a new site to that struct, the compiler may reorder the slot occupancy. Your existing contract stored uint128 amount at offset 0; after the revamp, that floor now lives at offset 16. That hurts. OpenZeppelin’s upgradeable contract patterns rely on append-only storage—never rearrange, never insert above existing variables. But the Solidity optimizer, when left unchecked, will happily re-pack a struct for efficiency, breaking the slot alignment your proxy expects.

The error is silent. No compiler warning, no trial failure on fresh deploy.

“We added one boolean to a packed struct and half our user balances turned into timestamps. Nobody noticed until the third migration.”

— Lead engineer, mid-size DeFi protocol, 2024

How to read a storage layout diff report

Stop guessing. The Solidity compiler outputs a storage layout JSON if you run forge inspect --output storageLayout or use truffle compile --all --save-metadata. Print that file for your current contract, then generate the same for the upgraded version. Diff them. What you're looking for is a slot number change for any state variable that existed in the previous version. A packed struct that used slots 3–4 might now occupy slots 3–5, pushing everything below it down by one. That shift corrupts every variable in every contract that inherits from the same base. Tools like @openzeppelin/modernize-safe-storage-diff automate this check: it reads both layouts and flags any reordering, insertion, or deletion in existing structs. I run this diff on every PR that touches storage. The output is boring when safe—identical slot layouts—and terrifying when broken: “Slot 7: changed from address to uint256”. That line means your modernize will silently corrupt data for every user who has ever interacted with that contract.

Most groups skip this. They shouldn't.

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One concrete trick: enforce a maximum struct size of two slots for upgradeable contracts. Why? Because a struct that spans three slots has a higher chance of catching a new field pushed into the middle by the optimizer. Stick to one or two slots, and if you demand more fields, split into multiple structs with explicit gap variables. Example: uint256 __gap_1; between the packed portion and the next logical group. That gap absorbs new fields without shifting existing data. The trade-off is wasted storage—typically 20–30 bytes per gap—but a one-off corrupted user balance costs more in support tickets and lost trust than 30 bytes ever will.

Testing upgradeability with OpenZeppelin trial helpers

Unit tests against the implementation contract will never catch this. You call a proxy modernize test. The canonical pattern uses @openzeppelin/hardhat-upgrades or @openzeppelin/upgrades-core for Foundry. Write a test that deploys the proxy, writes realistic data into your packed structs—say 1000 random user states—then upgrades to the new implementation. After the revamp, read every stored value and assert it matches the pre-revamp snapshot. If even one field returns a different value, the test fails. That's your safety net. I have seen crews skip this because “we only added a field to an internal function”—six hours later they were reverting the modernize. The OpenZeppelin helpers also validate that the new contract doesn't shadow any inherited variables, which is a separate but common pitfall. Run this test in CI. Make it a blocking check, not a manual step. The expense? Maybe ten extra seconds per test run. The cost of skipping it? A production outage and a hasty redeploy.

What usually breaks primary is the initializer modifier order—but that's a different chapter.

FAQ or Checklist in Prose

Can I ever pack a struct in an upgradeable contract?

Short answer: yes — but you need to earn it. The danger isn't packing itself; it's packing in the faulty slot or reordering fields after deployment. I've seen groups cram three addresses into one slot, save 20,000 gas on deployment, then watch their next refresh silently corrupt storage. That silence is the worst part — no hard revert, just flawed data. The trick is that proxy contracts (UUPS, transparent, or beacon) all store logic at a different slot than your structs. If you pack fields that must remain in the same order across upgrades, you're safe. The moment you add a field between two packed variables, the whole slot shifts. Your old proxy reads garbage from the new slot layout. The fix is brutal: you either keep the struct frozen forever (no new fields inserted mid-pack), or you version your struct with a new contract and migrate storage — a non-trivial weekend. One team I advised spent three days writing a migration script because they inserted a one-off uint16 between two packed uint128 values. That was a 25-slot rewire.

The real constraint is immutability of slot layout.

Not the data inside — just the physical position of each variable. If your packed struct sits in storage slot 7, every modernize must leave that slot's byte-level encoding identical. That means no adding fields, no removing fields, no reordering. You can append fields after the struct, but only in new slots — don't touch the packed slot again. Most upgradeable frameworks (OpenZeppelin's TransparentUpgradeableProxy, for example) enforce this via the Initializable pattern, but they don’t prevent you from redeclaring a struct with a different internal layout in a new contract version. That’s on you. A simple diff of your storage layout before deploy catches 90% of these disasters. Use forge inspect or Hardhat’s storage-layout tool, and check that no packed slot changes offset between versions.

What if I already deployed a packed struct?

You have three paths — none of them are clean. opening option: never revamp that contract again. Freeze the logic. Deploy a new proxy pointing at a new implementation that reads from the old, corrupt storage? That doesn't work because the new implementation expects its own slot layout. Option two: write a storage migration. This means a new contract with a migrate() function that reads all old packed structs, unpacks them, and writes into the new layout — slot by slot. It’s a one-way operation, costs gas, and breaks if your users don’t call it. Option three: use unstructured storage patterns (EIP-1967-style) to isolate your packed struct in a random slot via bytes32(keccak256("some.unstructured.slot")) - 1. This sidesteps the proxy’s linear slot collision problem, but now you’re managing your own storage namespace. I have seen teams choose option three and then forget to initialize the random slot — silent zeros everywhere. That hurts.

“Storage migration scripts are rarely tested until the hour before an modernize. That’s when you discover the off-by-one slot offset.”

— senior Solidity dev, after a 9-hour debug session

Checklist before your next revamp

Run this in order, not skipped. (1) Export your current storage layout — every variable, every slot, every offset. (2) Compile your new implementation and export the new layout. Diff them. Any slot where a variable changed size, moved, or disappeared is a red flag — especially in packed structs. (3) For each packed struct, verify that no field was added, removed, or reordered within the same storage slot. If you added a field to the end of the struct, it should land in a new slot, not shift existing fields. (4) Check that all storage gaps (__gap arrays) remain untouched. If you removed a gap variable, the next variable inherits that slot — chaos. (5) Simulate an revamp on a testnet fork with real proxy data. I mean it. Deploy the old implementation, populate ten structs with random values, modernize, and read them back. If any value differs by more than zero, your upgrade is broken — no amount of gas savings justifies corrupt data. (6) Write a one-off integration test named test_storage_consistency_after_upgrade. It should pass or you don't deploy. That test has saved me twice. The odd part is — both times the bug was a single missing uint256 that shifted everything by 32 bytes. Wrong order. Not yet. Fix it first. Packing structs saves gas, but only if you treat storage as graveyard — once buried, don't dig it up.