Revealing mistake: After Mr. Maxwell says that the local people refuse to work on the movie, Velma asks why. In her close up, her right pupil is horribly misshapen. Not round, but a wobbly, almost cloud like shape. (00:01:35)
Revealing mistake: Scooby knocks on a trunk and it knocks back. Fred is shown and tells Scooby to open it. But in that shot, Fred's face is so badly drawn, you almost can't even tell it's him if not for the clothes and the voice. (00:05:20)
Revealing mistake: After the Ape Man runs up the stairs, Fred presses the button to turn the stairs into a slide. But you can see as he moves his finger to the button, he never touches it. His finger hits the switch above the button. (00:19:25)
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.