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 this scene we see several Government Officials talking in a room. One of them, while talking, is playing a game. The game is to balance the lid of an ink bottle on the end of a ruler and over the ink bottle. When the ruler is flicked rapidly gravity would do the rest and the lid would fall into the ink bottle. Things don't work to plan and the lid falls on its side. In the next wide shot, the lid is right side up and a little distant from where it fell earlier.
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.