When she first meets Labeouf in her room at the boarding house, Mattie compares his fringed Texas clothes to those of a rodeo clown. Rodeo clowns did not exist until the beginning of competitive rodeos in the early 1900s, some 25-30 years later.
Before drunken shooting scene Rooster falls off his horse and the bottom of his boot is exposed. The boot has a modern Vibram sole with traction grooves. Boots of that period had leather soles with no grooves.
Near the beginning of the film, when Mattie is arriving by train in Ft. Smith, the camera pans across the ground and tracks to the train, revealing a pre-cast concrete grade crossing with steel edges, and then subsequent wooden grade crossings. These precast concrete grade crossings were not available in the late 1800s.
Early in the movie, there is a shot of a train, which is fitted with modern knuckle couplers and air brakes, neither of which had been invented at the time the action in the movie is to have taken place.
At the beginning of the movie (2:49) as the train pulls into town you'll notice there is only 1 train track, the one the train is on, rails on wooden ties with no platform under it. Cut to 3:03 and there is now a second track visible next to the train with wood crossing platform around it, and the train is on a modern set of tracks with cast concrete crossing plates are the track. In addition as the camera pulls back and down it is clear that the ground below the camera is paved black top. This town was ahead of its time.
At the end of the film, set 25 years after the main story, Mattie supposes that Labeouf (Matt Damon) would now be in his 70s, and closer to 80 than 70, meaning he had been at least 45, and closer to 55 when the manhunt for Chaney took place. But Labeouf's appearance was no more (and possibly less) than Damon's actual age of 40 when the film was made. The very precise Mattie Ross would not have made such an error.
When Ned captures Mattie, the pistol he threatens her with is a Remington New Army. A few minutes later, Ned reloads his pistol and he has a Colt Single Action Army. When he fires a shot to acknowledge Rooster keeping his end of the bargain, he is again using a Remington, however, when he pulls his pistol to shoot Rooster, he is back to a Colt SAA. While it is possible that Ned carried two revolvers, at no time is he seen with a second revolver on his person.
When Rooster Colburn is pulling cornbread out of his satchel and throwing it the air to shoot, you see that several pieces of cornbread fall out of his satchel and on the ground; however when they go the wide shot, no cornbread is on the ground.
Rooster throws a holster over the horn on his saddle before he and LaBoeuf have the argument after which LaBoeuf leaves them (the first time). During this argument, the position of Cogburn's holster on the saddle-horn changes several times.
When Mattie cuts the hanged man down from the tree and Rooster kicks him over so that he is lying flat on his back, his legs are crossed and his coat is half open. As Rooster stands over the dead man, Rooster's shadow falls behind him, away from the dead man. After the next scene, where the Indian on horseback approaches, no one has touched the dead man, and Rooster is standing in the same place. But now the dead man's legs aren't crossed anymore, his coat is fully open, and Rooster's shadow falls in front of him, across the dead man's legs.
In the court-scene at the beginning of the movie, when Rooster Cogburn is interrogated, the position of the eye patch, more precisely the band above his left eye, repeatedly changes position between shots.
In the final scene as Mattie is standing next to the gravestone of "Reuben Cogburn" the gravestone says "In Memory of". As she walks away and the credits begin, the gravestone now reads "In Loving Memory".
In the grocery store back room scene, the closeup of Rooster Cogburn brushing past the ducks shows the ducks hanging separately. In all wider shots in which the ducks can be seen, they are hanging in a tight group.
During the cornbread shoot, Cogburn shoots the first piece of cornbread with his Colt Single Action Army, in the next scene, when LaBoeuf shoots the cornbread, Cogburn is seen holding a Colt Dragoon, similar to the gun Mattie Ross has. It is NOT one of the Colt 1851 Navy's he carries on his saddle.
While Ned is holding Mattie pinned with his boot the position of her hat that fell off changes from being an inch from her head to completely missing. Also in the long shot it's leaned to the side but when she picks it up it's positioned straight up.
When LaBoeuf first meets Mattie Ross in her room at the boarding house, he is smoking a pipe. At one point during their conversation, LaBoeuf puts his pipe down on the table next to his chair. The smoke curling up from the pipe suddenly changes shape and position.
In the conversation between Mattie and the horse trader Col. Stonehill, he is seated, leaning back in his chair, with both arms on the armrests. Mattie speaks and the scene cuts to him saying, "You have no case." When he says this sentence, he's leaning forward in the chair, his right arm is extended out to his right and is resting on some papers on his desk. The scene cuts to Mattie speaking again and when it cuts back to him, he's leaning back, both arms on the armrests.
Just before Rooster falls off his horse after climbing the hill, he polishes off the small bottle of whiskey while on horseback. Any remaining whiskey would have been lost in the fall anyway.
When he gets up off the ground, there is about 1/3rd of the bottle still full.
When Mattie meets Colonel Stonehill for the first time, the left and right sides of his mustache do not match. One side is white and matches his beard, and one side is less gray with more blonde and doesn't match the beard. When she meets him the next time, both sides of the mustache match.
At one point in the film, Rooster says that they have a choice of heading north into the Winding Stair Mountains or continuing further west. The Winding Stair Mountains are 30-40 miles southwest of Fort Smith, meaning they would have to travel south to go up into the mountains, not north.
When Rooster and LaBeouf are talking about serving in the Civil War, LaBeouf says he served at Shreveport with Kirby Smith, in the Army of Northern Virginia. Smith was actually in command of the Army of the Trans-Mississippi.
In the night cabin scene with the two men by the firelight (and when camera shot is from a low angle) Jeff Bridge's eye patch reveals its upper half construct to be of a see through mesh and not solid black fabric. You can clearly see Jeff's eye.
When Mattie scales the tree to cut down the hanged man, the length of rope changes length between shots. When we first see him, he is hanging very close to the tree. In the next shot, he is hanging a fair amount lower. In the original screenplay, Rooster cuts the rope at ground level and the body falls about 5 feet, and then the rope snags. That is why Mattie has to climb the tree to cut him down. The scene of Rooster cutting the rope at ground level was filmed, but did not make it into the final cut. That is why the body is seen hanging several feet lower.
Mattie tells Rooster that her sister's name is Victoria. Later, in her letter home, she asks her mother to pinch Violet on the cheek for her. However, at that time "Violet" was commonly used as a nickname for people called Victoria.
When Mattie first encounters Rooster at the courthouse, she keeps standing in his way to prevent him from leaving the courthouse, but exit doors are shown behind him (which he eventually walks through). Most courthouses of this time (including the Blanco, Texas courthouse where this scene was filmed) were square-shaped and had exits on all four sides. Mattie was only blocking him from exiting the doors he was originally heading towards.
The goof items below may give away important plot points.
When Mattie is fetching water from the river and encounters Tom Chaney, she is carrying a wood bucket. Such an item would be very unwieldy and bulky to carry while riding horseback, and none of the characters is shown to be carrying one. More likely, Mattie would have used a canvas bucket, and this in fact, is mentioned in the book.
When Mattie Ross shoots Tom Chaney the first time and she falls backwards into the water we can see her gun the entire time when she falls and then rights her self, she could not re-cock her gun without the camera seeing it, however when the camera cuts to her standing her gun is cocked and she then attempts to fire it. This is impossible due to the fact that her pistol is a Colt Model 1848 which is a single action pistol, meaning she would have to cock it after each shot.
Toward the end of the movie when Rooster is on the ground and Ned Pepper is on his horse ready to shoot him, LeBoeuf is aiming to shoot Ned with his Sharps Carbine. He successfully shoots Ned, and then gets hit in the head with a rock by Chaney. Mattie grabs the carbine and shoots Chaney. The carbine that LeBoeuf used is a single shot breech load carbine. He can be seen reloading while he is talking to Mattie, and closes the breach shortly before being hit.
Le Boeuf uses a Sharps carbine for the long shot (over 400 yards) that kills Ned. This is not at all an impossible feat for a Sharps carbine. Furthermore, he would not have used a full length rifle, as it would have been bulky and impractical on horseback.