In any video game, being beaten, in any division of game (firefight, round, etc.), by a newb by a large margin, or being defeated by one of the least powerful weapons in the game when you have clearly superior weaponry.
Playing Halo 2 while you have the sword and are defeated by someone with a needler.
A long time player being defeated by their friend who has never played before.
Even more so when a combination of the two.