Not code, but I once pasted an event announcement and asked for just spelling and grammar check. LLM suggested a new version with minor tweak which I copy pasted back.

Just before sending I noticed that it had moved the event date by one day. Luckily I caught it but it taught me that you never should blindly trust LLM output even with super simple tasks, no relevant context size, clear and simple one sentence prompt.

LLM's do the most amazing things but they also sometimes screw up the simplest of tasks in the most unexpected ways.