Visible crew/equipment: In the scene where the knights are attacked by the killer rabbit outside the cave, you can see the boom mic at the top of the frame.
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: 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)
Audio problem: Robin's minstrel's hand doesn't hit his tambourine the first time we hear them sing.
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.
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.
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.
Other mistake: It's revealed that the Knights who say Ni can't say "it" (at least, after they become the Knights Who Say... that other thing), but they still say "it," most notably when they're telling Arthur to place the second shrubbery next to the first one - "When you have found another shrubbery, you must place IT here, beside this shrubbery..."
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: 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.