Trolling, mild griefing, and general bumblefuckery in TF2 (and maybe some other games) as long as it's with friends.
It doesn't matter what it is, as long as it get's the hornets nest stirred up, and we get to watch each other run and/or hide for our lives.
Or sometimes it's just doing random silly stuff to each other, or other people, wich are just passing by.
Sure, we have our standards regarding politeness. As noone of us spawncamp, or do stuff that's really meant / or likely to seriously ruin someone's day. But, when we're having fun, we're doing anything but playing the game, like it's meant to be played!