When Vernon Adams (Robert Duvall) first aims down on Maddox (Burt Lancaster) with his rifle from above, Maddox is riding away from him up a long draw. Immediately afterward, Maddox is still traveling up the draw and looks up and sees Vernon in front of him and above him aiming down at him. The positions switched 180 degrees.
During the opening scene when Bronson's rowdies tear up the town of Bannock, in two views of the local hotel, the town's name is spelled Bannock. Later, when Sheriff Maddox checks into the hotel in Sabbath, he signs in as a resident of Bannach.
When Laura is walking away from Vernon (seen from behind) she is carrying the pistol at her right side. Next scene (seen from the front), she is holding the pistol in front of her.
One of the characters uses the word "gunsel", a word generally acknowledged to have been coined by Dashiell Hammett in 1929.
When Maddox (Burt Lancaster) shoots the horse out from under Vernon Adams (Robert Duvall), the man who is thrown from the falling horse has a full head of hair, and is clearly a stunt double. Robert Duvall was totally bald on top in this movie. The stuntman even tries to hide the fact by placing his hand right on top of his head as he comes up, but the full head of hair is still visible.