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?
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.
The goof items below may give away important plot points.
During the final battle sequence, Sir Hector can be seen wildly hacking away at an enemy in close proximity to King Arthur, when he strikes Arthur on the back of his head with a full blow from his sword. Arthur, appearing momentarily stunned, grabs the back of his head but being faced with an opponent and film rolling, continued the fight sequence with his left arm up in the air to protect his head as he tried to move away from Hector who was still wildly swinging at the other actor - completely oblivious of the blow that he had inflicted on Arthur who was standing directly behind him.