Ditching ones friends to be assaulted by the Horde on Left 4 Dead or any other multiplayer game which requires everyones full teamwork and cooperation. Usually resulting in the deaths of all teammates but said ditcher. Also, betraying ones friends by lighting them on fire with any in-game incendiary weapons.
Anthony Left 4 Help last night while I was being attacked by the Horde, then decided "help" more by throwing his molotov directly at my feet and surrounding zombies.