Here's bug in my roguelike called Backpackless that drives me just aarrrrgh: I've forgotten to code the check for the arrows to end :(
The player can obviously abuse this bug to win (bows are pretty powerful weapons).
The most painful thing was I noticed the bug in the period when itch.io was down after the deadline, so I could've fixed it and uploaded the correct version, but I decided that it would be unfair so I didn't.