Factual error: The movie takes place in 1868, and every guy and his dog are wielding cartridge conversion Army, Navy, and even what appears to be Walker colts. Unfortunately, such conversions were not performed until 1869, after Smith and Wesson's rights to the Rollin White patent expired.
Revealing mistake: When the Chinese workers set the railroad tracks down they start driving spikes but it's plain to see there are no spikes to drive.
Revealing mistake: There are two scenes where Gideon is riding across the dry plain where there are vapor trails visible in the blue sky. Firstly, in the shot immediately after he has a drink and steers the horse's head, in the top left of screen. Secondly, as he is disoriented, he is trying to get his bearings, in one of the shots panning across the plain just before Madame Louise first appears.