Continuity mistake: At the beginning, Eastwood dismounts from his horse to rescue the calf lying in the river. His horse is left standing in the river with the water up to his knees. When Eastwood places the calf on dry land, his horse is standing nearby. The horse's legs are dry.
Continuity mistake: At the end of the movie when Cheyenne is shaving, while Harmonica and Frank are having their shoot-out, he cuts himself at the sound of the gunshots. When Jill tells him that he is a handsome man the cut is gone. (02:22:25)
Factual error: This movie is set in the pre-Civil War Old West. At about 01:06:50, a slave, who is hoping to be taken to Mexico (where slavery is prohibited), is trying to bamboozle the lady played by Shelly Winters into believing he has vast astrological knowledge. And he points at the sky and mentions Pluto (which hadn't yet been discovered then). (01:06:50)
Revealing mistake: At the beginning of the opening title sequence if you look behind Silence as he rides through the snow covered forest you can see what looks like a couple of skiers whizz by. (00:05:34)
Revealing mistake: When Buck and the gang ride away from the saloon in Dallas where JC made a few new 'friends', the footage is sped-up for a second just as soon as James Caan jumps on his horse. (00:05:50)