Ours is pretty simple, though it gave us a good laugh when we first encountered it: in our game, you use a projectile which can bounce off of walls, which is accompanied by a sound effect. However, due to an oversight in how we calculate collisions, if the projectile hits a corner, the sound is played multiple times in the same update, which results in an unexpectedly loud, distorted version of the effect.
Another one would be, to not duplicate code the enemies' bullets are actually just enemies who travel in a straight line. However, due to the way we code collisions between enemies (who are repelled from one another, so they don't stack together), this results in enemies being slightly pushed away by bullets (and not the other way around). We decided to keep that in as well, also gave us quite a laugh when we discovered, what is happening ;)
If anyone is interested in the game, here's the link: https://itch.io/jam/gmtk-2019/rate/462587