Continuity mistake: When the hangman puts the noose over Roy's head, he places the knot on Roy's shoulder so it hangs down the front of his body, as does Chon's. The camera then cuts to a wide shot of the gallows and the knots on both Roy's and Chon's nooses are suddenly hanging down their backs.
Continuity mistake: When you look at the bottle of whiskey during the Chinese Drinking Game, you can see the bottle's contents shrink and grow.
Factual error: During the train fight scenes, as they are hopping from wagon to wagon, you can see the wagons fitted with modern couplings to connect them together, rather than the throw-over hook and loop couplings that would have been fitting for the time the film was set.
Revealing mistake: When Roy and Chon are escaping from jail, Chon uses all his strength to bend one of the iron bars aside so that Roy can squeeze through. But, as Roy squeezes through, the iron bar behind him (which should be rigid) also visibly flexes, revealing that the bars are made of rubber or plastic tubing.