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: Before the ambush with the Indians, we see Buffalo Bill on a horse - only it's not him, but a stand-in. In the next shot we see Buffalo Bill closer up, and comparing the profile and facial structure of both shots, it's a different person on the horse to the actor that portrays Buffalo Bill. (Stand-ins were often used in older films when the actor was not available for filming for whatever reason.)
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.