Movie Goof (continuity error): When Rooster and Mattie are trying to get out of the snake hole, Rooster goes to pick up Mattie's father's gun. In that shot, his gun belt is suddenly missing.
Movie Goof (continuity error): La Boeuf puts the serving spoon back in the bowl and picks up his fork. The next shot shows him putting the serving spoon back in bowl and picking up a piece of chicken.
Movie Goof (errors made by characters, possibly deliberate errors by the filmmakers): Mattie Ross' gun is a Colt Walker, not a Colt Dragoon as stated by Rooster.
Movie Goof (miscellaneous): When Rooster hits La Boeuf with his rifle right after Mattie is captured, the rifle bends. It is obviously a rubber gun.
Movie Goof (anachronisms): Some of the paths Mattie, Rooster, and La Boeuf ride down have obviously been formed by automobile tires.
Movie Goof (continuity error): Near the end of the movie down in the snake pit, Rooster ties the rope around his waist, Mattie asks him to get her father's gun and when he turns to grab it, the rope is gone, when he turns back around to pick up Mattie, the rope is back around his waist
Movie Goof (continuity error): After he kills Frank Ross, Chaney kneels by the body to steal the money. Then Frank's head is on Chaney's left-hand side. In the next shot, Frank's head appears on the opposite side.
Movie Goof (continuity error): After Mattie rides her horse across the river, when Rooster and Le Boeuf won't let her on the ferry, her clothes are perfectly dry.
Movie Goof (continuity error): After La Boeuf succumbs to his head injury and falls off his horse, Rooster turns him over to look at his face. La Boeuf rolls over with his feet crossed. As Rooster and Mattie are riding off and look back at him, his feet are now splayed out.
Movie Goof (continuity error): When Mattie approaches the ferry pulling the horse, to join Cogburn and La Boeuf, her shadow indicates the sun position to the right. Subsequently, all of them are shown from behind, with their shadows indicating the sun position to the right again.
Movie Goof (continuity error): In the shootout between Rooster and Pepper's gang, the film reverses for a few seconds. Rooster's eye patch moves to the right eye, the bandanna switches sides, and his rifle and pistol change hands.
Movie Goof (anachronisms): Kim Darby's hairstyle is from the 1960s.
Movie Goof (errors made by characters, possibly deliberate errors by the filmmakers): Rooster mentions that he lived for some years in Cairo, Illinois, but he mispronounces the name of the town. The local pronunciation is KAY-row.
Movie Goof (continuity error): During the hanging scene, the song "Amazing Grace" is being sung. The scene switches to some kids on swings, and the line "How sweet the sound" is sung in the middle of another verse, then the song goes back to the verse that was being sung.
Movie Goof (errors in geography): When the characters in in Fort Smith, Arkansas, and after they ride into Indian territory, there are many shots of rocky peaks and snow-covered mountains. There are no such mountains within several hours' automobile drive of Fort Smith, and especially not west of the city; the Boston Mountains north of town, which are basically the foothills of the Ozarks, are green, tree-covered mountains.
Movie Goof (continuity error): When LaBoeuf and Rooster are discussing the venture at Chen Lee's, Mattie repeatedly changes position between shots.
Movie Goof (factual errors): After testifying before Judge Parker, Rooster picked up his gun and belt from the bench outside the courtroom. There were still several other gun belts remaining on the bench, clearly showing that anyone wishing to enter the courtroom must first check their firearm. However, when Mattie follows Rooster out of the courtroom, she is holding the white sack out in front of her, which anyone could see contained a gun. Under the rules of Judge Parker's court, Mattie should have also been required to check her firearm.
Movie Goof (anachronisms): Although the date on Frank Ross's grave indicates he died in 1882, the rifle Cogburn carries is a Winchester Model 1892 saddle carbine.
Movie Goof (continuity error): When Mattie emerges from the river, she is already dry.
Movie Goof (continuity error): The white markings on Little Blackie's legs and face change throughout the movie.