There are three hazards to make things more interesting. Missing this number with all three darts means the player's score at that stage is reduced to 0. This is the 5's. Missing the 5's with all three darts sends the player back to his beer, he's out of luck and out of the game. This is a bonanza for the player who gets it and a tragedy for the other players.
Shanghai is, in one throw, hitting a single, double and triple of any of the numbers in the game.
This feat wins the game there and then and no further darts can be thrown. A Shanghai is valid as long as the single is not the last dart to go in.