Mine Your Own Business - S1-E4
Other mistake: When Daphne falls down the mine shaft, Fred has turned his head around his body 180°.
A Night of Fright is No Delight - S1-E16
Other mistake: Velma asks Shaggy "How'd you get that green stuff on your hands?" but there is no visible "green stuff."
Other mistake: When the gang are swimming, a plant in the foreground moves along with them.
Mine Your Own Business - S1-E4
Other mistake: When Daphne asks "What do we do now?", her lips are higher than they should be.
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.