Corrected entry: The movie is set circa 1868 (e.g. mention of three years passing since being wounded in the Civil War), but a cash register is seen in the saloon. The first cash register wasn't introduced until 1879, and the "traditional" style seen in the saloon wasn't available until years after that. Source taken from http://www.moah.org/exhibits/archives/kaching.html.
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.