On custom edition on Coldsnap someone had managed to glitch into our base and was in a longsword dropping nukes on us. 221 kills before we shot that !@#$% down.
You think that's bad? No glitches, but a spawn camper managed to headshoot me like, 18 times in a row, before I even had the chance to move. Once he ran out of ammo, I killed that guy in like, 5 seconds, with a grenade and a few well-aimed pistol shots.
Still, I was mega-pissed off and haven't played a single online FPS game since.