Mattie and her horse are completely dry right after swimming across the river. This same error occurs in the original film.
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.
The date for the Wild West Show on the flyer is July 18. When Mattie shows up there , Cole tells her Rooster died 3 days ago. The date of death on his gravestone is Aug. 12.
When Marshall Cogburn is shooting the last piece of cornbread, his coat kicks up and knocks his hat off his head. In the next shot of him, his hat is back on his head.
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".
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.
LaBoef refers to his horse as a "white appaloosa". It is clearly a red roan appaloosa, with red spotted spotted white haunches. An appaloosa's color is determined by the color of the main coat of the horse, since many appaloosas have white haunches. A white appaloosa is white overall, usually with spots of another color.
True Grit is set in 1873. Issac Parker was appointed judge in the US District Court for Arkansas' Western District in 1875.
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 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.
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 snakes Mattie sees in the cave are clearly rat snakes from their markings, but she calls them rattlers and you can hear rattling in the background.
When LaBoeuf confronts the gang outside the cabin, with Mattie and Rooster on the hill under cover, one of the gang throws a rope over LaBoeuf. LaBoeuf then throws himself sideways as if pulled off his feet by the horse, but the rope is slack and doesn't become taut at all - hence no pull. He clearly just jumps sideways.
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.
Mattie had to cut the rope for the hung man to fall from up in the tree when clearly the rope must be tied near the ground.
As the tree with the hanging man has no low branches, there's no way that Mattie could have ascended the tree unsupported as Cogburn directed.
When Mattie and Rooster hear the warning gunshot in the distance and are waiting for someone to appear, a car is visible in the close-up of Mattie on her horse.
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.
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.
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.
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.
When Ned Pepper is pinning Mattie Ross with his boot in The Winding Stair Mountains, after the first pan down at Mattie and pan back to Ned you hear the director yell "Action".
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.
After Maddie is snake-bit and Rooster and she leave for the doctor, they pass by the seemingly perfectly sound horse of either Ned Pepper or one of his gang. Rooster could easily have taken that horse with them in tow so as to make better time when little blackie started to break down.