Revealing mistake: In the scene with the French taunting they attack the castle. Shortly afterwards they run away and hide on the hill slope. When Arthur is trying to lie down he nearly guts one of the knights with his sword and just after that when he gets his sword in front of him you can see that it is a fake wobbly sword. (00:27:54)
Revealing mistake: The side of the beard of the three-headed knight's left head (their right) comes off just before we see that Robin has run off.
Revealing mistake: When the black Knight is sitting on the ground with no arms or legs, the ground right next to him bends down, indicating his legs were buried.
Revealing mistake: After King Arthur cuts off both the Black Knight's arms and both his legs, as Arthur rides away and the Knight is struggling on the ground, as he turns sideways you can see both his hands, held behind his back, and the depression in the ground where he is standing in a hole. This is how the effect was achieved.
Revealing mistake: During the bit where the Knights are attacking the killer rabbit, in a few shots, most noticeable in an over-head view, you can see the wire that pulls the rabbit puppet along.
Revealing mistake: When Lancelot is running back down the stairs during the wedding, you can see the sword he uses to cut someone is bending. (00:57:45)