Factual error: When Wyatt (Kevin Costner) becomes the sheriff/deputy/policeman of Wichita, Kansas, you can plainly see mountains in the background - there aren't any around there.
Factual error: Maverick and the rest of the poker players throughout the movie are using plastic poker chips. You can tell by the sound they make when thrown. Poker chips during that time were made out of wood or ceramic.
Factual error: The girls stop at a grave site, and the marker indicates the husband had died 3 years earlier, in 1888. Yet, later on, Cody tells Frank that they're going to the "Oregon Territory". Oregon had been a state for over 30 years by that time.
Factual error: During the battle scene the voiceover says that it is February 1915. The battle being portrayed is obviously the Second Battle of Ypres because it involves Canadian troops and it's the first time that gas was used in the war. The actual battle didn't take place until the end of April 1915.
Factual error: In the saloon at night, Ben Wheeler says, "I can't afford another Workers Comp claim". Workers compensation wasn't started in the United States until 1902. (00:10:25)