Continuity mistake: We see Morgana for the first time as she stands in the back of the wedding and turns around to look at Merlin. The order of the guests in front of her does not change but the banners above their heads are entirely different as she approaches Merlin in the next shot. (01:00:45)
Continuity mistake: Lancelot says to King Arthur, "I sought only not to harm you" before he dismounts and they fight with swords under a tree. If you look closely at the tree there are no marks on it before they start fighting, King Arthur strikes one blow and as Lancelot pushes him away there are several marks on the bark of the tree. (00:49:30)
Other mistake: At the siege of Leondegrance's castle, when they show the knights on top of the siege tower moving to the wall, one knight in particular puts a cigarette in his mouth through the breathing slit in his helmet before crossing onto the wall.