Revealing mistake: In one of Pickett's charge scene the bayonets are obviously rubber.
Revealing mistake: When Wyatt Earp and the actress, whom he marries later in the movie, meet on horseback, they go on a fast paced ride that concludes after going down a steep hill. If you look at the actress' feet, you will see she is riding side-saddle - a very difficult feat considering the riding they do. If you look closely you can see her real leg, safely on the other side of the horse. The side-saddle leg must be fake.
Revealing mistake: When Alferd runs from the saloon with a lynch mob behind him, we can see a line in the dirt ahead of him that serves as a marker for where he is supposed to run.