Don't Fool With a Phantom - S2-E8
Other mistake: When the wax phantom appears behind Scooby and Shaggy the man who supposedly is impersonating him in the suit is in clear sight directly behind Fred Daphne and Velma.
Jeepers It's the Creeper - S2-E3
Other mistake: There is a scene where the gang is the home of the Bank President, just after dropping off the injured bank guard. At one point the Bank President is standing behind an end table of some kind with a lamp on it. Even though there is nothing underneath the table to block your view you cannot see the lower part of the Bank President's leg. It looks like it has been cut off.
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.