Plot hole: It seems unlikely that: 1. The curator, who's short and chubby could fit into that suit of armor. 2. How he could be strong enough to lift up the table Scooby and Shaggy are hiding under at one point, 3. That the museum has no security guards who might catch the curator and his thugs during their art forgery scheme and 4. There are eyes shown moving in the suit of armor when the gang's talking to the curator despite the fact he's the Black Knight Ghost.
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 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.
Continuity mistake: When the gang are in the Malt Shop at the end of the episode, Scooby puts straws together to steal Shaggy's drink, and succeeds until Shaggy cuts the straw. But after Velma says "That closes the mystery!", Scooby starts drinking through his straw and we cut to the other four drinks on the table. This time, Shaggy's glass is completely full.