Revealing mistake: Early in the movie, there is a visible circular, plastic looking effect on Gideon's arm as he removes the bullet with the knife. This is more noticeable around his shoulder when he cauterizes his wound with the hot knife.
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.
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.