[kibwen]

One of the best bugs I've ever seen was in the game Dungeon Crawl Stone Soup. It's a roguelike, and a venerable old one at that, so the codebase is quite tangled and convoluted, with logic for everything in every corner. One of the worshippable gods in the game is Xom, god of chaos, whose effects are just as likely to kill their worshipers as to help them (frustrating for those who might actually want to win, but always good for a laugh). And somewhere in the main game loop was some bit of logic that was only ever supposed to activate while the player was actively worshiping Xom. Guess how this was accidentally implemented:

  if (you.religion = GOD_XOM) {

This actually made it to the live test server, where player save files are automatically upgraded to the most recent build, so anyone logging in immediately found themselves infallibly worshiping the god of chaos, every single tick of in-game time (renouncement is futile). Chaos indeed!