Halo: Combat Evolved

Bug: On the map "Danger Canyon" it is possible to go through the stone wall behind the base and pop up on the enemy base. If friendly fire is off, then you can have someone ram you with a vehicle at high speed while you're standing against the wall. Or you can get in a Ghost and drive sideways towards the wall, if you hop off of the Ghost at the right time, then you should end up partially inside the wall, if you jump a few times into the "nothingness" then you may pop up on the enemy base! Or fall to your doom.

Trivia: Try completing the game on legendary, you will get a funny outro for your time.