Other mistake: When the girls leave camp, if you look closely it is obvious that one of the girls (the one in the distance) is a double who does not look like Susan / Sharon at all.
Other mistake: When they're roping the mustangs, the "stallion" doesn't have the genitals of a stallion.
Other mistake: Just before Miss M sees the murder on the passing train, there is a shot of the express train overtaking hers. At the relative speeds, the express would have passed her window in 4 seconds, but she sees the murder for nearly 14 seconds.