Continuity mistake: During the fight at the OK corral, Doc Holiday has a double barrel shotgun but he shoots it three times, once in the air to spook the horse, once again to shoot the guy behind the horse, then the scene changes and he shoots another guy with the same gun without reloading.
Continuity mistake: The scene where Ed says "there's three of them over there" referring to the Cowboys, but when the shot is shown there's actually four Cowboys.
Factual error: Billy Breakenridge in real life was a large man who was almost certainly not gay. He certainly wasn't effeminate as the film makes him out to be and he was known as a good shot and a tough individual.
Deliberate mistake: When Wyatt comes out of the Orientalist and joins his brothers in the street to tell them he's acquired a quarter interest in a game, a herd of cattle is passing them in the street but there's no dust kicked up by the cattle, when in reality there would be a lot of dust on a dry desert street in Arizona.
Continuity mistake: After shooting up the wedding near the start, they are sitting at the table eating the feast. As the camera shows Curly Bill's plate the food on it keeps changing.