Factual error: The movie takes place in the early 20th century (1910s or 1920s or so). When Dean and Jerry are riding in a RR cattle car with its doors open you can clearly see parked automobiles from the 1950s through the open door of the train car.
Factual error: Two grave markers at the end, approximately one hour and 27 minutes into the film, have the date of November 7, 1889, on them. The script made it clear that the climactic gunfight between Glenn Ford and Broderick Crawford came on a Sunday. Ford was called out of church services, for example. According to The World Almanac and Book of Facts, November 7, 1889, was a Thursday, not a Sunday.
Continuity mistake: The opening scene states the date as April 10th 1865, the day of the train station robbery. Siringo tells Vance at the school money-raising event that the robbery took place 8 weeks ago, but when Clint is buried at the end, his death is April 25th 1865, only 2 weeks after the robbery.