The door to Bruce Barkow's office is locked for the meeting with Beau Dorn. Dorn leaves through without unlocking it.
None of the pistols or rifles recoil when they are fired. Also, the rifles don't eject any empty cartridge cases when they are cocked for the next shot.
When Rafe shoots Taggart, he jacks another shell into the chamber and then shoots the other guy on the roof, but no empty shell is ejected from the chamber.
When Beau Dorn is riding out of town at sunrise to kill Rafe, he stops in the street to look at Anne Rodney, and they are both brightly lit from the side. However, the sun is only rising at Beau's back and is not above the horizon yet, the sky is still fairly dark, and there isn't enough natural sunlight to light them up that way.
Shortly into the movie, Selleck's character Rafe Covington sits around a campfire with two companions. Covington is smoking a filter cigarette. Later he orders supplies and requests "rolling tobacco." The film is circa 1876. Filter cigarettes are not invented until the 1950's.
When Dewey comes around the bar to prevent the marriage, it is reflected in the mirror.
When Barkow comes from behind the house he tells Rafe that he is trespassing illegally. This is an error that an educated man would not make. Trespassing is illegal. So, it's incorrect to add illegally as there is no such thing as legal trespass.