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."
Factual error: The bad guys are vanquished, and Wyatt boards the train and heads off into the sunset. Two problems with that scenario: from Los Angeles, heading West puts you into the Pacific, and Wyatt didn't have to take the train, he lived on West 17th Street, in Los Angeles.