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.
Audio problem: When Bunny (Ramon) is, as Zorro, spelling out "Zorro" on the wall with his whip, the number of whiplash sounds does not match the number of marks in the wall.
Factual error: When Clint Eastwood first goes to the barber shop, the barber very deliberately sharpens his razor the wrong way, flipping the blade with its sharp edge against the strop - this would instantly dull the blade. No real barber would make this mistake, but it's a common movie error.
Factual error: The movie depicts Col. Nelson Miles at the signing of the Fort Laramie Treaty which was signed April 29, 1868. Col. Miles is shown wearing a Civil War Medal and an Indian Wars Medal. The first Civil War Campaign Medal was issued in May 1909 and the blue and gray ribbon shown in the film was not in use until August 1913. The first Indian Wars Medal was issued July 1908. The ribbon shown in the film with two dark bands was not in use until 1917. Obviously, Col. Miles could not wear two metals that have not been created yet.
Continuity mistake: Near the end, Sam Clayton is nearing the finish. The audio leads you to believe that the horse is breathing VERY hard, but when the camera shows the horse, the nostrils are NOT flared as they would be on a horse that has been run to exhaustion (or even down the block). Also, the people filming this scene obviously have no clue about the nature of sweating in horses. They showed "lather" all through the horse's mane. The ONLY places horses will lather while sweating is where something is rubbing, i.e. underneath the saddle, the bridle or breast collar, where the reins rub on the neck or between their back legs.
Continuity mistake: When Mitch is trying to find the hand, Glen grabs the map. When he grabs the map there is a hole in it, but before he sets it on fire, the hole is gone.
Continuity mistake: This is actually in a deleted scene on the DVD: Jones shoots a mountain lion at night, and it's seen to be completely dark out. The two other men go out to look at it, but when they're walking across the snow, the sky is suddenly pale blue and there's light just below the horizon. It looks like it's just before dawn or just after sunset. But when they reach the dead mountain lion, the sky is suddenly black again, and it looks like it's the middle of the night.
Audio problem: The Rabbi's horse is bucking in reaction to the rattlesnake, and all the while the Rabbi is yelling in Yiddish - but his mouth is not moving.
Visible crew/equipment: At the beginning of the film when being rescued from the quicksand you can see someone's legs and hand behind the horse, holding it. (00:10:00 - 00:11:00)
Continuity mistake: The story takes place in 1901 and Rose is still shown as a toddler. She would have been a teenager by then.
Factual error: During the final shoot-out, when Raf shot the man on the roof as he enters town, the wrong flag is displayed - a 48 star flag, but the time setting is the 1880's, when there were fewer states.
Revealing mistake: When Captain Quincy Wyatt interrogates a captive Seminole Native American by threatening him with a poisonous snake, the native is safe because there is a protective piece of glass between him and the snake. The snake's reflection is quite obvious on the glass panel. (01:14:40)
Revealing mistake: When The Indians launch their final attack (towards the end of the film), on a old outpost held by a small group of soldiers, Ward Bond is hit by a arrow which enters the right side of his chest. In the next shot you see Gregory Peck removing the arrow from the left side of Ward Bond's chest.
Factual error: When the trappers first encounter the Rickarees, the bow cannon is discharged, creating an explosion near the Indians, who promptly turn and ride away. Immediately after, a second shell explodes near them. It would be impossible for a single gunner to sponge out and reload with cartridge and ball in the space of 10 seconds.
Continuity mistake: In the prosecuting attorney's speech in the first courtroom scene, the judge's house of cards is no more than 2 levels high (a base level with a peak) [0:2:20]. In the close-up shot of the judge, the house of cards changes to 3 levels high (2 base levels and a peak)[0:2:35]. When the scene focuses back on the prosecuting attorney, the house is back to two levels of height[0:2:39]. (00:02:20)