Continuity mistake: Indians are on a hilltop and a jet vapour trail can be seen in the sky.
Factual error: Near the end of the film Butch is complaining about the living conditions they have to endure - jungles, swamps, snakes, night work - and Sundance sarcastically retorts "Bitch, bitch, bitch!" In 1908 the term meant just what it literally means: "Female dog." It did not adopt its current meaning of "complain" until much later. At the time the film is set - outside the context of "female dog" - it was considered to be a serious obscenity, and it would not have been used to describe something as ordinary as someone moaning about his living conditions.
Visible crew/equipment: In the last scene, when they are riding alongside a river, shortly after the harmonica player changes songs, there's a closeup of John Wayne and appearing from the right on the other side of the river, a pickup truck appears, and then they quickly cut to different characters. (01:56:55)
Visible crew/equipment: In several scenes there are rectangular arrays of studio lamps visible, reflected in the actors' space helmets.
Other mistake: Throughout most of the film, Mattie Ross' gun is correctly shown to be a Colt's Dragoon .44 caliber revolver. However, in the scene where Tom Chaney is looking down at her from the top of the well into which she had fallen, the gun in his hand is a Colt's Model 1860 Army. Presumably, when he, too falls into the well after being shot by Marshall Cogburn, the gun goes with him. When Cogburn retrieves the weapon before helping Mattie out of the pit, the gun is once more a Colt's Dragoon.
Continuity mistake: At the end of the town collapse, a sincerely drunk Ben dives head-on into a tub where a lady (in her underclothes?) is just enjoying a bath. Propelled by Ben's momentum the tub with both passengers slides down the slippery planks and splashes into the river, while Ben rearranges himself into a comfortable sitting position between shots. If I wasn't completely mistaken also the lady changes her outfit, since the embroidery is missing when they float along. (02:23:35)