Continuity mistake: After Evans was shot near the end of the movie, Ben Wade jumped out of the train to look at him. If you look over Wade's shoulder, you will see that Nez (the Apache) had already arrived at his spot, and Campos (the Mexican sharpshooter) threw Wade's gun belt to Charlie Prince. After the movie cut a couple of times, Nez was shown walking up, throwing Wade's gun belt to Campos, who then threw it to Charlie Prince.
Continuity mistake: In the opening scene of the robbers attacking the armored stagecoach, there are two guys at the back of the stagecoach manning the Gatling gun. One of the two gets shot and tumbles off the coach into the dirt. When the scene switches back to the perspective of the pursuing bad guys, there are briefly two guys at the Gatling gun again. Then, in another quick cut, it's back to one.