Mine was in Halo 3 matchmaking. There was this high-schooler who was harassing me for kicking the crap outta him and his friend in a game. It was kind of pointless though, because he's a random person over LIVE, and it only makes him look significantly dumber.
I can't quote what he said because of the MASSIVE vulgarity, but it was pretty darn bad.
What's the worst person (or people) you were ever paired up with through LIVE? Please share your experience here.
Thanks aplenty to Phobotech for the great user icon.