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.
Ned Pepper: What's your intention? Do you think one on four is a dogfall?
Rooster Cogburn: I mean to kill you in one minute, Ned. Or see you hanged in Fort Smith at Judge Parker's convenience. Which'll it be?
Ned Pepper: I call that bold talk for a one-eyed fat man!
Rooster Cogburn: Fill your hands, you son of a bitch!
Emmett Quincy: I don't remember any Ned Pepper.
Rooster Cogburn: Short feisty fella, nervous and quick, got a messed-up lower lip.
Emmett Quincy: That don't bring nobody to mind. A funny lip?
Rooster Cogburn: Wasn't always like that, I shot him in it.
Emmett Quincy: In the lower lip? What was you aiming at?
Rooster Cogburn: His upper lip.