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: Sean Connery's character is the Provost Marshall for the Presidio, he would not be wearing the MP sleeve placard as he is in the scene at the Officer's Club. He also wouldn't be running around wearing a Field Jacket with the collar up and it unzipped. You wear the jacket correctly or not at all.