Sometimes, when playing a game, you come across bizarre situations. Programming errors, that weren't intended to happen, messing up the game's graphics, physics, or A.I. These errors are called bugs, or glitches. A bug is more like a typical error, like random freezing, or crashes. A glitch is an aspect in-game, that creates unintended gameplay and strange behaviour. This allows you to experience a game in a way, the developers didn't want you to, which can result in very funny situations. Some good examples I found online: Mercenaries 2, 2008. "...Heeyy..."
Far Cry 2, 2008
Pro Evolution Soccer, 2009