Factual error: The Henry Repeating Rifle seen in 1865 is impossible, because it's the "King's Patent Henry Rifle" which wasn't out until 1866. The original Henry had a loading tube under the barrel, not one on the side of the breech. Plus the ammo used is the larger, more modern rounds.
Factual error: When Lt. Dunbar is preparing for his meeting with the Indians, you can see the rubber sole of his boot as he is putting it on. There is also a stamped logo on the boot heel. Rubber bottomed boots did not exist during the Civil War.
Factual error: We see and hear geese flying overhead toward the end of the film, a sure sign that winter is approaching, but the birds in the shot are sandhill cranes, which do not "honk" like Canada geese.
Factual error: When Lt. Dunbar gets to the Fort, they show a dove in the rafters. It is a domestic Ringed Turtle-Dove or domesticated Barbary (African Collared) Dove. They likely did not have those doves at the Fort, but rather should have been a Mourning Dove. Rock Doves and chickens were common domestic animals brought for food.
Factual error: In a scene where Dunbar is at the fort and a Henry 1860 rifle is lying on a firing port, there appears to be a long cartridge similar to 45/70 or.50 Govt with the rifle. The Henry 1860 rifle at the time used .44 Henry cartridges which were significantly shorter. (00:41:55)
Factual error: When Dunbar is leaving for his assignment, the officer holds a gun to his head to commit suicide. He's facing the camera with the gun screen left and a window screen right. If he shoots himself in the head from that position as implied, the bullet would have gone through his head and blown out the window, which the viewer sees intact after the shot when Dunbar looks back at the sound.
Factual error: When Dunbar saves the native American boy from the charging injured buffalo he jumps off, asks if he's all right then makes two repeated shots from his Henry rifle without cocking the lever. Only special safari guns did that, at a later date.