Ask HN: What mistakes did AI make in your codebase?

An Ask HN poster says frontier models including Opus 5.5, Astra, Sol 6.1, Fable 5.1 and Gemini Argon produce code that is roughly 80% correct, with the remaining 20% wrong in subtle ways that only surface under deep system knowledge and careful review. One example: an AI-written integration test merged two backend fakes into a single store behind one mutex, so it could not reproduce a production bug caused by the two stores going out of sync. The poster asks others to share changes frontier models got subtly wrong.