In one scene near the end, one of the townsfolk refers to Olklahoma as a state, "...not in this State". Olklahoma only obtained Statehood in 1907, long after the Daltons' demise.
The film's climax shows Emmett Dalton being killed in a shoot-out during an attempted bank robbery in Coffeyville, Kansas. In reality, Dalton survived the shoot-out and went on to write the book that this film was based on.