TF2: A Spy not being cloaked, not being disguised, shooting his gun at a turret that was at least 10 feet away that wasn't disabled.
LOL: Can't remember the heroes' name, but it wasn't a Tank, I think it was some sort of buffer/healer or something, walked straight to one of the defense towers, and stood there, motionless. After getting killed, he called us, his teammates, cheaters, and left. What?
NEW! Super Mario Bros. Wii: A friend of mine being an asshole, playing the game by himself. There were three of us, and as soon as the game started, he started running to the finish line, like there was some sort of challenge or something, and he'd die, PLENTY of times, and blame us, his teammates because "you couldn't keep up" or "you were dead" (FYI, if your teammates are dead, and then you die, you lose the game, if at least ONE of them is alive, he/she can revive everyone else).
I swear, I died 5 times in a sole stage because he kept moving to the edge of the screen, and killing me.
Also, he kept all the items for himself, because "I want to have the highest score!". What kind of crap is this?