When somebody teabags me. It's just so... ridiculous. And I still have fond memories of the time I made an enemy team so angry they had to both teabagged and dance on my corpse (They were good, so I knew I couldn't beat them. Instead, I started killing myself every time I respawned, making the match drag almost to the time limit. Finally I was too slow and they got me, and went into a 'bag and dance frenzy).