Factual error: The movie begins at the end of the Civil War, 1865; it then moves ahead 7 years, to 1872 in Wyoming. Several characters are wearing gun belts with cartridges on the belt. The pistols are Colt Peacemakers probably .45 cal. The problem is that the gun and cartridge were not introduced until 1873 and would take several years to become commonplace. In 1872 the percussion cap and ball revolver, which did not fire fixed cartridges, was still in use.
Continuity mistake: When Wyatt is confronted by Ed Ross in the bar after refereeing the boxing match toward the beginning of the movie, he has a pistol in his belt when throws the cue ball at Ross. Wyatt still has the pistol in his belt when he takes Ross' gun and holster and straps them on. But when he and Sally walk outside right after, the pistol that was in his belt is gone.