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.
Factual error: Campbell is armed with what is described by Wayne as a "buffalo gun", a large bore rifle known for its power and substantial recoil. Campbell shoots a wild turkey with the gun and causes the meat to be severely damaged. Wayne comments, "Too much gun" emphasizing the power of the gun. However, when Campbell shoots the turkey with this "buffalo gun" there is no visible recoil.
Continuity mistake: Near the end of the movie, after Rooster and Mattie's horse has collapsed, Rooster begins to carry Mattie. You can see that Mattie's blouse has come untucked from her skirt. However, once Rooster has acquired the wagon, Mattie is laid across his lap and her blouse is now tucked back into her skirt.
Factual error: There are no snow-capped mountain peaks anywhere near Ft. Smith, Arkansas. They appear to have chased Ned Pepper all the way to the Rockies, 500 miles or more away.
Factual error: On the second day of the trio's journey from setting out from Ft. Smith Arkansas the scenery jumps to the landscape of the Black Mesa near Santa Fe New Mexico. The credits substantiate that indeed some scenes were shot in this area. Ft. Smith is 729 miles from Santa Fe. The terrain near Ft Smith is mostly wooded for hundreds of miles.
Continuity mistake: When Rooster is walking out of the court room and meets Mattie on the stairs, his bow tie is different, or tied differently than the one he is wearing when he is being cross-examined.