Continuity mistake: When the gang are at the Malt Shop, on the table Fred and Velma's glasses are filled with milkshake. When Fred puts down the newspaper, both his and Velma's glasses are now empty.
Continuity mistake: When Shaggy is sitting down on the beach, the umbrella behind him is blue. When he runs into the umbrella a little later on, it is now coloured red.
Audio problem: When the gang (except Fred) talk about unloading the Mystery Machine, there are no lip movements whatsoever.
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.