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)
Continuity mistake: The witch's hand changes position from the shot where she is weighed to the close-up of her saying "It's a fair cop" or something like that.
Factual error: In the scene inside the Cave of Death, the priest reads the 'Last Words of Joseph of Arimathea'. As he reads them, his eyes go from left to right. If it was Aramaic like he said, he would read it from right to left.
Visible crew/equipment: In the scene when Sir Robin meets the three headed man, you can just about see the boom mic above them. It is about an inch below the top of the screen.
Continuity mistake: When Galahad is being looked at by the two doctors at Castle Anthrax, his gloves are on. When the doctors lift his tunic to look at the injury more closely, he pushes the tunic back down with now bare hands. They are not shown removing his gloves, and he can't have removed them since he was too busy watching the girls examining him.
Continuity mistake: When the Three Headed Knight asks Sir Robin "What do you want?", you can see one of Sir Robin's minstrels puts the mouthpiece of his recorder into his mouth, but in the next shot, the music plays and he puts the mouthpiece back into his mouth a second time.