What’s the hardest bug you’ve ever had to track down in a Solidity project?

We’ve all run into at least one Solidity bug that took way longer than it should’ve.
Sometimes it’s a storage layout issue, sometimes it’s proxies behaving weirdly, an inheritance quirk, calldata alignment, math edge cases, or a subtle security assumption that only shows up under certain conditions.

Curious to hear from other devs what’s the toughest or most confusing bug you’ve had to debug in a smart contract, and how did you finally track it down?