Factual error: At Fort Winslow, the soldier assigned to take Corporal Woodson to the infirmary is referred to as "corporal" and wears a rank insignia of one chevron. A single chevron was not authorized as the insignia of any rank in the U.S. Army until 1920, and even then, it denoted a Private First Class, not a Corporal.
Factual error: Corporal Woodson (the black soldier on Yellow Hawk's escort detail) should not be present, since in 1892 black cavalrymen could only serve in the segregated 9th and 10th Cavalry Regiments. The unit depicted in the film is the 2nd Cavalry, at the time an all-white regiment.
Factual error: While briefing Captain Blocker on his mission, the colonel references the Department of the Army. The Department of the Army was established in 1947; in 1892, when the film is set, it was called the Department of War.