The Lakota and Arapahoe tribes were allies. They sometimes lived side by side and intermarried. For the Lakota (Gall's tribe) to hold an Arapahoe captive would have been unconscionable.
During the the big gunfight towards the end of the movie when the corporal (Warren Oates) gets shot in the head, the red special effects shot comes in from the left side of the screen and sticks to the right side of his forehead.
After Kelly shoots Sayapi and walks over to his body, Sayapi's chest rises as he inhales.
The opening title card refers to Luther "Yellowstone" Kelly. In the bar scene, however, Major Towns calls him Lester Kelly.
When Kelly & Harper run to the aid of Wahleeah that had fallen from her horse, neither of the two men were carrying a firearm, a big mistake for frontiersmen.