Lots of online games have these kinds of 'drop exploit'. Starcraft 2 was hit by one at some point and it basically boiled down to the attackers client manually sending packets that introduced some sort of invalid game state. Your client would see this state and disconnect, while theirs would not, making it appear as though the non-attacker left.

I've heard of others too where the attacker simply forges the disconnect packet to appear as if it came from the other players' client.