Stupidist thing a friend ever did in a game? Well once a mate came round to play Melee and chose Pichu as his character despite it being his very first try, punted his ass halfway across the map, least ways when he wasn't catapulting himself off the edge with the spastic jumping Pichu has.
Oh, and there was a time on L4D when one of my friends was standing holding a medkit, just about to heal himself, when a tank spawns, so he panics, runs at the tank flailing the medkit before getting punched in the head, turns out he did 15 damage to the thing! Goooooo MEDKIT!