The only time I can recall being banned from a server was back in the beta days of Counter Strike. I had just joined the game and was skulking about with a MAC-10 with a friend. I turned around just to check behind us right as the remaining three members of the terrorist team rounded the corner. In a blind panic, I waved the MAC-10 in their general direction spraying bullets and accidentally got three head shots in the time it took to discharge a magazine. I was kicked and banned because cheating was a more plausible explanation than extraordinary dumb luck.