In the final battle scene, the Woads are shown using trebuchets. Catapults would have existed in Europe at this time, but not trebuchets. The traction trebuchet did not reach Byzantium from China until sometime in the late 6th century, over 100 years after the time of the film, and the counterweight trebuchet (the one used in the film) did not reach Europe until the 12th century (over 700 years after the time of the film).
In the scene where Arthur and his knights first encounter the Woads, they are trapped by entanglements of barbed wire. The extrusion technology necessary to produce wire was not available at the time. Barbed wire was developed on the American frontier in the 1860's - patented by Joseph Glidden in 1874.
The swords are from well after the setting of the movie. Roman troops would have been equipped with the spatha. The swords are a medieval design which did not appear until at least six hundred years after the time setting. The other hand weapons of the "knights" are equally incorrect.
Arthur and his knights ride horses using stirrups. Stirrups were invented in China around the 4th century and were not seen in western Europe before the 8th century, two or three hundred years after the events depicted in the film.
When warning Bishop Germanius before going on the final mission, Arthur makes reference to "papal armies". Papal armies did not come into existence until many hundreds of years later in the late middle ages.
When the knights are going back to the fort after the first fight, their positions in the group change between shots. After talking with Bors and Gawain, Lancelot rides forward, but in the next shot he is at the back of the group.
During Arthur and Lancelot's discussion before they leave on their mission, the lighting falls on the left side of Arthur's face in close up, and the left side of Lancelot's, despite the fact they are facing each other. In the wider shot moments later, the right side of Arthur's face is illuminated (as it should have been all along).
Just after the end of the first battle with the Woads, Bors draws back the curtain of the carriage to look in on the Bishop. As he does so, the leg and boot of a crewmember is visible inside the carriage.
During the final battle scene the camera pans all around the inside of the wall. If you watch as the camera pans the very top of the wall, where the stones are staggered, you see another camera moving on a sort of zip line as the battle rages on.
The Saxons invade from the North in the film, but in historical fact, they invaded from the South, as the first Saxon Kingdoms in Britain were in Kent. Also, the Saxons did not initially invade; they were invited to Britain to help defend against raiders from the North sea, later known as the Vikings. The Battle of Badon hill took place approximately 80 years after the Roman withdraw from Britain in 496 A.D and Baden Hill itself is nowhere near Hadrian's wall, it is much farther South, most likely on the hills surrounding Bath.
Throughout the movie, the politics of Rome are portrayed as if the Pope ruled the empire: A bishop, rather than a military officer, is sent to deliver the knights' discharges, and the decision to send them on one last mission is made by the Pope alone. In reality, both the Western and Eastern Empires were ruled by Emperors. The Pope, on the other hand, was not even the head of the entire church (let alone an empire) at this time - he was still just the Bishop of Rome. While he was highly regarded by the rest of the bishops in Christendom, it would be another 600 years before the Pope was recognized as the head of the entire church.
Pelagius did not advance a theory of political freedom, but resisted the doctrine of original sin, arguing that one was able to perform good works and achieve salvation by sinlessness alone without requiring spiritual Grace. It was declared a heresy of the Roman Church in 418 A.D.
During the final battle, these is no one to open the gate in Hadrian's Wall, it just seems to open and close on its own. However, the extended edition shows that at least Jols and Ganis stayed behind, possibly to work the gate mechanism.
After Arthur is slashed in the neck during the ice battle, his neck is bandaged after Dagonet's funeral, although you can still see the blood. In the sex scene with Guinevere that night though, there is no bandage and not even a scratch on his neck.
In the final battle sequence, after the first wave of attack is slaughtered, the last survivor comes out of the door at the gate and collapses. Right before he does the door behind him closes. When we see the gate from the Saxon's side the door is open. Then it's closed on the Roman side and the door is open again in the next scene.
The film begins in AD 467, and most of the main action takes place 20 years later, in roughly AD 487. However, included in the narrative is the official withdrawal of the Roman army from Britain actually (which happened in AD 410), one of St. Germanius' visits to Britain (which occurred in both AD 429 and AD 447), Cerdic and Cynric's arrival (which happened in AD 495) and their deaths (which happened in AD 534 and AD 556 respectively).