Revealing mistake: After Jim's horse is shot from under him while running down the really steep hill, and Jim later regains consciousness, before he turns and finds his horse dead, if you watch closely you can see the horse still breathing.
Revealing mistake: In the final scene as Doc tell Billy good-bye if you look up to the top right hand corner of the screen you can clearly see a jet trail in the blue sky. Certainly not possible in those days.
Factual error: The events in the film take place in mid-1929, as evidenced by the first Academy Awards being presented in May. However, Wyatt Earp went to the happy hunting grounds in January of that year.