Revealing mistake: Jack Elam gets bothered by a fly crawling over his face (nice stunt, actually). When the fly moves over the bench, just before he catches it with the muzzle of his gun, look at the hind legs. You'll notice that the fly is dead.
Revealing mistake: When Jill offers herself to Cheyenne she puts down the kettle on the table so hard that the lid falls off. If you look closely (slow motion helps) you see the lid dangling by the knob between her fingers before she puts the kettle down.
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.