I think mine was in TF2. I was playing a payload map (sorry, I'm kinda new to TF2, don't remember the name; it was the final map, has a bridge on it right before the first checkpoint (also, that's the only checkpoint, at least i think so)). I was of defense and had played like 10 h of the game. I liked the Engineer so i built a sentry to a random spot and upgraded it to level 3 ASAP. When the round started, it started killing the enemies. And killing, and killing. In total the sentry killed 14 enemies (to me, it's a whole shitload), a big part of the time i had to run back from the base since i died.
But the sentry kept on killing. I think the enemy thought our team had retreated and kinda rushed it. And my sentry caught them unaware. Anyways, i was maddened with happiness that time.