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. (00:15:25)
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: 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)
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: 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: 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: 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.
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: In the cave scene, as the knights are arguing about the meaning of the Castle "Aaaargh", the face of a fake Sir Robin is revealed when Lancelot turns his head. That's because Eric Idle, who plays Sir Robin, had to play brother Maynard.
Revealing mistake: When Arthur chops off the Black Knight's first arm, you can see his blade does not slice through the arm, but rather just bumps into the arm, which proceeds to fall off, revealing a fake prop-arm was used.
Chosen answer: It's kind of both. He says "Blue", then realises he got it wrong and starts to say "No, yellow!" but then screams as he is thrown into the pit, so it's like "Blue. No, yell. Ooooooohhhhh!"
Sierra1 ★