During his first encounter with John Carter in the Thark village, Tal Hajus's broken tusk repeatedly switches sides between shots.
When John Carter and Dejah Thoris are getting married, her eye color switches from blue to brown and then blue again.
After John Carter has saved Dejah Thoris from the collapsing airship, the Tharks hand him their wager.
He then takes off the necklaces and other jewelry except for a flat white neck ring.
The camera switches to Dejah and back to Carter and the neck ring is gone.
When Ned is given his uncle's private journal, he first turns the book so that he can open the lock with his right hand. After the attorney leaves, he turns it another time, just to put it in the same position again.
During the fight after the wedding, John Carter grabs two Zodangan swords on the ground. In the battle with Sab Than, they get slashed by Sab Than down to the hilt. After he jumps away, John Carter gets caught by Sab Than and falls. When Sab Than holds his ninth ray sword to the face of John Carter, there is only one hilt of the cut swords in John's right hand, the grip of a Helium sword instead of a Zodangan sword.
Even though John is able to jump higher and further than he could on Earth, he's still subject to the same gravity of Mars as everything else. Yet somehow when Dejah is falling, he is able to leap downwards at a faster rate than her and catch her before she hits the ground.
Carter's Confederate record states that he was "decorated six times, including the Southern Cross of Honor." In fact, the Confederacy never issued medals and only added a few names to a "Roll of Honor." The Southern Cross of Honor was a memorial recognition created by the United Daughters of the Confederacy in the late 1890s, 30 years after the Confederacy was disbanded.
At the beginning of the film, Powell tells Carter that the U.S. Army needs his help in dealing with the Apaches and recites his assorted achievements while serving in the Confederate Army. Under the 14th Amendment, anyone who participated in the Rebellion was ineligible to later serve in any position of honor under the U.S. government. Not even a Presidential pardon could change this. However, the disability could have been lifted by a two-thirds vote by both Houses of Congress, something not likely in Carter's case.
The moons of Mars, especially the smaller one, are not spherical, and they're always shown close together in the film, even though they actually have very different orbits.
The sun is so far away that its light shines at the same angle upon Mars and its two moons; this means that all three bodies always have the same phase. Yet looking up from the nighttime wedding, the 'tops' of both moons are in broad daylight.
In the beginning of the film, John Carter is told that to send a telegraph; he must choose a minimum of 10 words. The letter is shown as only containing eight words. However, telegraphs have always had a certain number of words as a minimum cost. It cost however many cents per word, but the minimum was 10 words that it would cost, not the minimum number of words that can be sent.
When John Carter is trying to pull the chain out of the rock in the arena, the ring fastening the chain has an obvious gap for him to unhook it easily. The gap disappears in the next shot.
The white ape's blood is blue, so it makes no sense that its mouth, gums, and tongue are light red.
Powel tells that John Carter that "Uncle Sam" wants him for army duty. The character of Uncle Sam first referred to barrels of beef supplied by Samuel Wilson during the War of 1812, it wasn't until political cartoonist Thomas Nast in the 1870's began popularizing the image of an Uncle Sam, which then appeared on Army recruiting posters in 1917.
When John Carter transports to Mars at the end, he is portaging into an unknown situation. Matai Shang has had several years to manipulate the situation on Mars, including masquerading as John Carter to manipulate Dejah as well. This is not really the typical Disney happy ending; a possible explanation is Disney wanted to make room for a sequel.
The Thern watching over John Carter would have no reason to believe that he actually found a medallion in the Orchades, since he knows the whereabouts of his peers, and would know that no Thern died or lost their medallion in this location.
Throughout the movie, Dejah Thoris' name is pronounced Deh-jah. According to Edgar Rice Burroughs' own notes, the name is supposed to be pronounced Dee-jah.