Plot hole: Immediately before the attack on Pony-That-Walks' village Capt. Brittles states that it's 12 minutes to midnight,and orders the attack. The attack is a surprise attack, yet all the warriors pile out of their tipi's fully clothed, painted and with their eagle feathers braided into their hair.
Factual error: In the very beginning of the movie, the narrator says, "Pony Express riders know that one more such attack (referring to the Custer Massacre) would mean a hundred years before another wagon train crosses the prairie." Then they show a Pony Express rider supposedly carrying the news. But the Custer massacre happened in June of 1876, and the Pony Express went out of business in 1861.