Factual error: During the confrontation between Little Bill and English Bob outside the barber shop, all the Deputies cocked their guns - for emphasis. Then Mr. Beauchamp reaches into his bag to show that he only has books, the Deputies cock their guns again. With the rifles they have this would eject the shell that was in the barrel and cycle a new one. It doesn't.
Factual error: When the dead cowboy is brought into town and Little Bill is getting briefed, one of his deputies remarks that "Witherspoon" will not sell them any more 30-30 shells unless they pay. The movie takes place in 1881, but the 30-30 cartridge wasn't developed until circa 1895. (01:36:45)
Factual error: The tension rod in Ned's house is plastic - wrong for the time.