Visible crew/equipment: In the scene where the knights are attacked by the killer rabbit outside the cave, you can see the boom mic in the top of the frame.
Continuity mistake: When King Arthur is fighting the Black Knight, there is a fire behind the Black Knight that is lit in some shots and nearly out in others.
Continuity mistake: In the scene where Sir Galahad is about to enter Castle Anthrax, it's pouring like there's no tomorrow. Although when he falls against the door and is about to open there is no rain, and no Niagara Falls pouring down the door. As a matter of fact, it's pretty dry on the door and almost a completely clear sky.
Revealing mistake: When King Arthur is fighting the Black Knight, when he is about to chop off his last leg we can see the Black Knight jump slightly, after he has done this we can see his leg fall before King Arthur has touched it with his sword.
Visible crew/equipment: In the scene when the knights are fleeing from Castle Anthrax, there is a crew member visible on the left side of the screen, just when they hurry along the outside wall from the castle.
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.
Continuity mistake: After Sir Lancelot has gone on a rampage at the wedding party, the bride is first seen with a clean face. Then in the close-up she has loads of bright red Pythonesque blood coming from her mouth and running onto her chin. In the next shot, the blood has disappeared again.
Continuity mistake: When the knights have built the rabbit and given it to the French they hide behind the top of the hill with woodland behind them. Then when they rabbit is launched at the you see a shot of them running uphill through clear ground.
Continuity mistake: When the rabbit kills the first knight that is sent to kill it, the rabbit gets blood all over its mouth. However, when the knights charge the rabbit, in the shot of the rabbit watching them, the blood has disappeared. It reappears in the shots where it is slaughtering the other knights.
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.
Continuity mistake: When the Green Knight is charging at the Black Knight with the axe, there is a shield leaning against the tree behind him. Then in the shot right before King Arthur cuts off the Black Knight's first arm, the shield disappears.
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.
Continuity mistake: When Brother Maynard is reading the inscriptions on the wall Sir Robin is hiding behind Sir Lancelot, but whenever there is a shot from behind the group Sir Robin disappears.
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.
Continuity mistake: When King Arthur threatens to say "Ni!" to the old crone, the left side of his chain mail coif is behind him; before and after that cut, both sides of his coif are draped over his shoulders.
Continuity mistake: When King Arthur arrives and the Black Knight is fighting someone else, the Black Knight stabs his victim straight through the helmet between the eyes. When he removes his sword the sword is through the helmet across his nose, a movement of 90°.
Continuity mistake: When King Arthur and his knights go to fight the killer rabbit and then run away, Sir Galahad left his shield behind, but one second later when they're regrouping, he has it again.
Revealing mistake: When the Black Knight kills the other knight by throwing a sword through his head, you can see that the blood is coming from the sword, not the wound itself. Likely the actor had a blood tube in his hand, and let the blood run down the sword and splatter onto his face.