Factual error: The evil man request 100,000 pieces of gold for ransom. Each gold piece looks to be about 10 ounces, and 100,000 of those would make the entire thing weigh about 31 tons, but 2 guys are carrying it around in a little chest throughout the movie.
Factual error: Just before we see Roy O'Bannon's gang for the first time as they are preparing to rob a train, we see a shot of the train winding along track in a very picturesque valley. However, the tracks are paralleled by very modern telephone poles with several wires on them. If this was truly the old west, the poles would have had only a single wire on them for the telegraph service.
Factual error: During the train fight scenes, as they are hopping from wagon to wagon, you can see the wagons fitted with modern couplings to connect them together, rather than the throw-over hook and loop couplings that would have been fitting for the time the film was set.