What a Night for a Knight - S1-E1
Continuity mistake: When Shaggy puts the ladder on the wall near the Museum Rear Entrance, you can Fred is standing by an open door of the Mystery Machine. In the next shot, he is now standing near Shaggy.
What a Night for a Knight - S1-E1
Continuity mistake: When the gang say "Professor Hyde White!" in the library, you can see Shaggy has his hand on Fred's shoulder. When we Fred and Velma in the next shot, he doesn't.
What a Night for a Knight - S1-E1
Continuity mistake: When the gang are driving along in the Mystery Machine, you can see Velma is sitting by the window. When we see a full view of the front seat, she is now sitting between Shaggy and Daphne.
What a Night for a Knight - S1-E1
Continuity mistake: When Mr Wickles is standing by the crate containing the Black Knight armour, you can see the lid is resting on it. But in a close-up of Mr Wickles, the lid is missing.
What a Night for a Knight - S1-E1
Continuity mistake: When the gang discover an empty suit of Black Knight armour in the truck, you can see the helmet is beside the wheel. When Daphne notices a piece of paper by the wheel, the helmet is nowhere to be seen.
What a Night for a Knight - S1-E1
Continuity mistake: When we see the truck at the beginning of the episode, you can see that the label on the crate is completely clear. When the Black Knight opens the crate, the label now has writing on it.
Chosen answer: A ghost wouldn't have to keep any food in the fridge - they don't need to eat. So it is one thing that proves the ghost is not a ghost.