John Carter is shown to be a widower because of the War Between the States. However, in the book, John Carter's own words state that he never had time for romantic involvement, except on a temporary basis. Dejah Thoris is the only woman he ever loved.
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 of 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 9th 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 an Helium sword instead of a Zodangan sword.
Martian gravity is 2.7 times weaker than Earth's, which means that a human on Mars would be able to jump 2-3 times higher or farther than on Earth. Carter, on the other hand, is able to make jumps some hundred times greater even with another person on his shoulders.
Even though John is able to jump higher & 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's able to leap downwards at a faster rate and catch her before she hits the ground.
At the beginning 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 sun is so far away, that its light shines at the same angle upon Mars & the two moons; this means that all three bodies always have the same phase. Yet looking up from the night-time wedding, the 'top' of both moons is in broad daylight.
On two occasions, Dejah is falling from a great height when John makes a soaring leap, and catches her at its apex when he's traveling horizontally. Doing so instantly cancels out her downwards momentum - when either they should've both gone into a sharp dive, or else she would be killed as if she'd just hit 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.
In the beginning of the film, John Carter is told that to send a telegraph he must choose a minimum of ten 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 ten words that it would cost, not the minimum number of words that can be sent.
The goof items below may give away important plot points.
When John Carter first tries to leap over the white ape, the chain linked to his ankle drops him short about halfway across the arena. A couple of minutes later, after his chain gets tangled around the ape, wrapping around its enormous body several times, the amount of free chain looks to be nearly the entire length of the arena.
When Dejah Thoris is being prepared for her wedding she is wearing a traditional Zodangan dress and red lipstick, but the lipstick color is inconsistent when she asks him to stay and fight for Helium.
When he does not answer and hides his ring hand she turns away. If you look closely you will see that her lipstick has now been expertly reapplied and is a lighter color, almost peach.