Visible crew/equipment: In the scene where the judge is releasing Clint Eastwood and telling him that his story checks out he calls him over to the window to watch a hanging. As the guards walk down the scaffold steps on the ground under the gallows there is a pile of rope and next to the rope is a modern electrical box of some type. It's silver with with louvers (vents) in it.
Revealing mistake: Near the end, Clint Eastwood is sneaking up on a house at night. The old guy (Ed Begley Sr) shoots at Eastwood repeatedly until his gun is empty as Eastwood dodges from tree to tree. Just before running directly to the house, there is a strange black circle that completely covers Eastwood for a few seconds. It is very obvious and no reason why it should be there.
Factual error: In the scene where people are arriving in town for the hanging, a family are told they have tents set up in the Boondocks. "Boondocks" is a Philippino word that entered the English language when the Philippines was a colony of the U.S. The movie is set 9 years before the Spanish American war so the word didn't exist in the English Language during the period of the movie.