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. (01:42:20)
Revealing mistake: There is a scene where one of them jumps on a horse, and you can see one of the crew holding on to the reigns as the horse throws its head back.
Visible crew/equipment: In the shot where Dirty Steve is on his horse, he asks, "Hey Dog, did you see the size of that chicken?", and then he fires his shotgun. You can see someone's hand come up to grab the reigns of the horse. (00:43:25)
Factual error: When Billy The Kid is taking a bath and dictating a letter to Doc, intended for the governor, he says he is in the Mexican village of Juarez. This town was actually named "Paso del Norte" at the time, it wasn't until 10 years after the events depicted in the film that it was renamed "Ciudad Juarez."
Other mistake: In the final gun battle when the Regulators are trapped in the attic of the burning house, one of the characters backs into the brick chimney that's coming up through the floor. You can see the entire chimney move when he backs into it.
Continuity mistake: Near the end when they're entering the Mexican village, Billy's coat disappears as he rides through the gate. (01:17:45)
Visible crew/equipment: When Buckshot Roberts is going into the outhouse there is a sequence of shots that show Billy the Kid, Dick Brewer and Dirty Steve hiding behind a pile of logs. In almost every shot where these three are seen behind the logs you can see a camera tri-pod leaning against a fence. The sequence lasts over a minute so you have several chances to plainly see it. (00:47:05 - 00:48:15)
Factual error: When Irish Eyes are Smiling. It wasn't written until the early 1900's by Ernest Ball. He died in Southern California in a theater in Los Angeles in the 1930s, well past the death of Billy the Kid.