Other mistake: When Fred hears footsteps, he says "Into the closet." But the closet has stairs and passageways inside.
Other mistake: When Shaggy is on the ground after being struck on the head by a vase, the inside of Scooby's eyes are brown instead of white.
Other mistake: When the gang are reading the book in the library, the page they are on says "2", despite the fact that they are halfway through the book.
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.