God, this is the worst one that I can recall:
I was leveling a priest, and when I got to around ~54, I specific queued and got Dire Maul. I was the healer for the group, and I was more than friendly, and out of nowhere either the tank or dps (because in the end, they both ended up ganging up on me) started insulting me, mostly because of my server. I ignored him at first, but eventually he just started getting extremely nasty with the insults. When they missed a boss, I tried to lead them back in the right direction, and then one of the other group members started ganging up on me. Just absolutely terrible insults that I did nothing to deserve. I left the group because I couldn't take it anymore.