When did an AI coding assistant break something silently in production?

Code refactoring tools are amazing until they are too confident. I had an LLM clean up an unused configuration variable across a repository and it assured me every single reference was gone.

Turns out it completely ignored a dynamic string interpolation in an old logging script because it wasn't a direct match.

The app ran fine for a week until that specific error log finally triggered and crashed the background worker. Now I don't trust any automated cleanup without running a strict global search first.

What's the sneakiest bug an AI tool ever slipped past your tests?