Revealing mistake: When the guy on the horse pulls out his sword, he has trouble getting it out of the sheath at first and it bends severely. It is clear that it is not a real sword.
Other mistake: Early in the movie, Sir Gawain gets on his horse, in a full suit of armor, by himself. Later, he is hoisted onto his horse by what looks like an early design for a crane. Armor is heavy and Sir Gawain would have to be hoisted every time he wanted to ride with armor on.