Revealing mistake: A horse gallops through a field and you can see a telephone pole.
Continuity mistake: In the scene at the end when King Arthur's funeral raft is floating away, you see white smoke coming from the wood at the foot of the raft. Cut to the shore, and the flaming arrow is lit and shot. Cut to the raft, and the flaming arrow hits the now smokeless raft in the spot where the smoke was. Cut to the shore, then back to the raft, where the fire begins to grow on both sides of the raft, nowhere near the now smokeless and flame-less spot where the flaming arrow hit in the previous shot.
Revealing mistake: In the raid scene where we first meet Malagant, a four-sided wooden tower is shown on fire, and then it collapses from the damage. However, the tower's front legs don't buckle and as it falls, the burning tower appears to be completely intact -- it has simply been pulled down by ropes that are partially visible from the front.
Continuity mistake: When they are in the forest in the rain after they escape from the bad guys castle; they have a back and forth moment where Lancalot's hair on his ear changes from shot to shot. When the camera is behind him you can see a thick rope of wet hair across his ear, and when the shot changes, the ear is clear of any hair.
Continuity mistake: After the town in the beginning is ransacked and burned, the leader of the pack is giving his speech about who is their leader. He carries a sword with serrations on one side towards the hilt. A shot of his horse is shown and the serrations of his sword are pointing out at the crowd. The shot switches to behind him and the serrations are pointing towards him.
Continuity mistake: When Guinevere is being taken to Arthur they are stopped by a felled tree. But when you look out the front of the carriage, there's no tree on the road. Also, if the tree was really blocking the road, how could the horse and carriage suddenly charge off and easily get around this tree?
Continuity mistake: Closely watch the horses that are being used throughout the movie. The horse ridden by King Arthur and the one ridden by Maligant are one and the same. Also, the bay horse ridden by the stableboy is the same horse used to transport the kidnapped Guinivere to Maligant's castle. This would be okay, but both these horses are very distinct-looking, and it's painfully obvious that they're being shared.
Other mistake: When Malagant visits the Round Table, he places his sword at his old position but never retrieves it before he leaves.