The Dunwich Horror expansion of Arkham Horror includes a card called Joining the Winning Team. Basically it's a mission where you have to sacrifice an ally at a number of places. On completion, you win the game and all other players lose the game.
I'm aware of the fact that this is nigh impossible to complete (as of this writing, the Arkham Horror Statistics page says that only 11 of their nearly 13,000 recorded games have ended by its completion), but in the rare event that you might be able to pull it off, can the other players do anything to stop you? Can they attack you or anything?