Just before Mickey Simpson and his group break through to Randolph Scott and his group of trapped men, Scott blows out the candle that's illuminating his dark prison. When the film cuts to the view from the rescuers point-of-view shot, light can be seen through the hole.
The ink stains from Susan's fingers on Jeff's coat change direction and intensity of color between shots.
In the scene in Crocker's office about nine minutes in, Edgar Dearing's cigar shifts from his left hand to his right and back again.
When the bandits push the freight wagon over the cliff, the dead driver's body cannot be seen.
In the scene where the train is taken over near the end of movie, alternating shots of the cars show modern couplers and air brake hoses on the cars appearing and disappearing. The time period was before these were invented.