Abomb ( or animation bomb) effects the animations in game, by freezing doors, fireballs or other spell effects. It happens to characters with long amounts of gameplay. Generally 200+ hours.
But it doesn't happen all the time, not sure what the requirements are for it to happen. It happened on one character at 300 hours, yet another character I have is going 500 hours with no slowdown in sight. Yeah, I said 500 hours.