Factual error: John Blair rides for the pony express, which operated until October, 1861. However, all of the handguns in the movie are cartridge Colts, which did not come into existence until 1871.
Continuity mistake: In the scene where Buffalo Bill is about to take aim, his left hand is on the metal ring on the rifle. In the next close up of Wild Bill Hickok we can now see Buffalo Bill's hand has moved to another position further down from the ring and in the third shot, his hand is now covering the ring. (01:00:10)
Factual error: This movie takes place post-civil war, and Fisher, in his first discussion with General Cortinas refers to the "Republic of Texas" being ripe for the taking. Texas ceased to be a republic in 1845, when it joined the United States.