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 Wyatt and the others leave two cowboys hanging in front of the Dragoon Saloon, you can see that neither man actually has a rope on his neck. As the one man swings around, on his vest you can see the outline of the rope going straight down into the harness that's holding him. (01:25:35)