Revealing mistake: This mistake occurs in the beginning of the film where the bandits rob the train. When Jackie is standing on the coal (or was it wood?) car you can see the train is passing a modern sign (you can see it on the right).
Revealing mistake: When Alejandra comes to John's room the first time you see distinct t-shirt marks on John's upper body, his arms as tanned as his neck and face. This in spite of the fact that he wears long-sleeved shirts throughout the movie (except, of course, during his nightly encounters with Alejandra). (00:49:30)