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: 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.