When Arthur accepts Lancelot's invitation to joust, Arthur's squire carries three lances. Arthur takes a lance from Lancelot's rack and uses this for his first attack. He goes to his squire to collect a lance for the second attack and his squire only has one; what has happened to the other two?
Distance between Mordred and Arthur in the spear scene.
When Lancelot is injured and Merlin heals him, Guinevere holds Lancelot's hand up close to her but in the next shot, their hands are resting on his body.
When Perceval is about to defend the queen in combat, he and Sir Gawain are shown bearing shields with the same device. As heraldic devices were supposed to be unique to an individual, this should not be possible. Further, the device shown is that later attributed to Sir Lancelot, therefore Sir Gawain would never use such a shield.
Full plate metal armor was not invented until about the year 1350, and not used in Europe until much later than that. Numerous such anachronisms (use of stirrups, weaponry) can be excused as being faithful to Sir Thomas Malory's book, which followed 15th-century conventions of making historical characters behave in a contemporary way.
During the final battle scene against Mordred, the background audio track of men yelling and swordplay is clearly a re-tread of the Leon De Grance castle battle. In the final battle scene, one can clearly hear the "throw the rope" line that Merlin yells to Arthur from Leon De Grance castle battle, as well as the yell from Arthur as he jumped from the castle into the moat. (00:37:02 same as 02:88:18, 00:40:12 same as 02:09:58).
When Merlin creates a fog, the prolonged mist from Morgana's mouth is clearly coming from a pipe/tube next to her cheek. Her mouth is closed at times and the thick mist still continues.
The music being played for Igraine's dance is too advanced (use of syncopation, counterpoint) for Sir Thomas Malory's 15th century, much less the late 5th/early 6th century when Arthur is supposed to have lived--and since it matches the knights' beat and Igraine's movements so exactly, it can be supposed to be part of the scene and not merely incidental music (as, later in the movie, O Fortuna is).
In the scene where Excalibur is returned to Arthur before the final battle, a camera, camera man, and sound man along with several lights and other crew can be clearly seen in Arthur's armor.
During Merlin's speech to the knights, just prior to Arthur's announcement of the round table, etc., the hose supplying the gas for his flaming staff can be seen at its base.
After casting his spell on Uther, Merlin turns toward the camera, showing the reflection of the cameraman and camera in his helmet.
During the wedding scene, the camera is reflected in Arthur's armor.
When Arthur leaps off the castle wall to unhorse Uryens, he falls straight down, but Uryens is by the moat 10 or 15 feet away from the castle.
Boom mike reflected King Arthur's armour.
Arthur jumps from Leondegrance's castle wall to take Uryens off his horse, and then appears to be trampled by that horse.