Continuity mistake: When Daphne suggests leaving the painting room through another door, the Black Knight appears and you can see he has red crest of hair on his helmet. But after a quick cut, we return to the gang and the crest on his helmet is missing. It re-appears when he comes to the painting that Shaggy is hiding behind.
Continuity mistake: When Scooby Doo and the gang are pursued by the invisible ghost with a candle, they hide in a room and Scooby blows out the candle and shuts the door. You can see the door has a handle on the left hand side, but when we see the gang in the room in the next shot, there is no handle on the door.
Continuity mistake: When Velma loses her glasses, the Black Knight trips over her and lands on the guillotine. She mistakes the Black Knight's growls for Shaggy's sore throat, and pours some cough mixture on to a spoon. When Shaggy enters and runs up to Velma, she is wearing her glasses again.