I wrote code for industrial automation years ago (think assembly line machines). I was reviewing production code and found a stupid bug and fixed it, then reinstalled. The motors moved incorrectly - I don't recall if that was the time it smashed glass everywhere, but "fixing" the code definitely broke the program. I could not figure out why...but due to time constraints I sadly had reinsert the bug to put the machine back in production.

Some nights that still bothers me.