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.
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.
Add timerabid anarchist
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: When King Arthur tells Sir Bedevere that he is not saying "Ni!" correctly, Bedevere is looking directly at Arthur. When the camera angle changes to behind Bedevere in the next shot, he is now looking at the Old Crone, and in the shot after that, he is looking at Arthur again.