Although I mentioned it on the Headset thread, I thought it was particularly annoying when a few of us Chewers had a clan match (I believe it was Capture the Flag) against another clan. Time ran out and we lost 1 to 0. At that point the other clan started saying things like, "Y'all should bring your game back to the store and ask for your $50 back." Yeah, your one whole point really shows your dominance in the game, jerks.
At that point we decided we would be assholes. However, being the nice guys we are, we had trouble coming up with good asshole lines. I think the best we managed was, "Hey, you are a bad bad man."
Again, even when we go up against other clans, I've not heard ANYONE say anything bad post match. We might make a little fun of someone's gamer tag over the Team chat line, but that's about as bad as it gets. And hey, if you go by the name BadAssSk8r, you deserve a little ridicule.