Factual error: At the beginning of the movie, in the conversation betwenn Frank James and Robert Ford, the term "gunslinger" is used a few times. However, the term "gunslinger" didn't come into use until the 1920s (the movie is set in the 1880s).

Factual error: When Bob Ford opens the box under the bed, on top is an issue of "The Jesse James Stories" weekly, which was published from 1901-1903 - fully twenty years after the events of this film.