Factual error: I normally wouldn't bother with this sort of nitpicking, but this film specifically claims to be historically researched - and it's full of historical blunders. For a start, the film is set as the Empire withdraws its last troops from Britain - which was in 407 AD. Now Artorius Castus was a real Roman officer who really did command Sarmatian foederati at Hadrian's Wall, but he died around 200 AD. Cerdic was a real Saxon warlord who did go raiding the Britons with his son Cynric, but he did this in the early 500s. Pelagius really was tried for heresy, but he was acquitted and died of old age; the trial was a decade after this setting, and in the fifth century you couldn't be executed for heresy anyway. Also in the fifth century the Pope had no authority over Imperial troops. I could go on and on but that will do for now.
Continuity mistake: In the scene where the knights are sitting at the round table near the beginning of the movie one of the knights has a plus sign shaped scar on his forehead. In the next shot,the same knight now has a sideways cross shaped scar. The scar changes almost every shot throughout the scene.
Continuity mistake: During the battle at the end of the movie, the first wave of Saxon infantry is sent through the open door in Hadrian's Wall, and then it closes behind them. Mayhem ensues. All but one of the soldiers escapes after the Wodes fire arrows at them while the Knights ride them down. The single survivor staggers through the opening doors (which open just a crack, enough for the guy to slip through), and then stands there, and in that shot we see the doors behind him are now closed. An instant later, it cuts to the hillside where the Knights are standing on their horses looking down at the wall, and we see the doors both standing wide open. Scene cuts back, and we see they're closed again.
Deliberate mistake: When Arthur follows Guinevere to the meeting with Merlin, her and the trees' shadows point at the viewer. Only Arthur has two shadows, one points to the left. The source of the moonlight is below the next ridge, and it's bright like a floodlight. Later, when Arthur is talking to Merlin, his face is always illuminated, even after a 180°-turn.
Audio problem: When the Woads attack in the beginning of the movie, after the knights have pretty much won the day, Bors hold up his fist knives, sticks out his tounge, and yells a sort-of war victory yell which sounds rather comedic. At the end of the movie when the knights and their horses charge into the Saxons out of the fog, you can hear Bors' exact same yell once again.
Factual error: At the very beginning of the movie, the voice-over tells us it's around 300 AD. We see Roman soldiers, carrying shields and standards with the Chi Rho symbols. Those weren't introduced around 300 AD, but much later. Legend has it that emperor Constantine ordered his men to paint the symbols on their shields before a battle in 312, but Constantine's Arch in Rome, which depicts his victorious soldiers, doesn't show any of these symbols and the symbols in the movie are made of iron, not paint.