Continuity mistake: Scooby finds a clump of ape hair stuck in the wine cask. It cuts to the gang, then back to the cask as they walk up and suddenly the clump of hair is twice as thick. (00:16:45)
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)
Continuity mistake: Just before Shaggy goes into the secret passage way, he sits on a stone, and his hair has somehow turned yellow.
Continuity mistake: When the gang 1st arrives at the beach. Fred stops the Mystery Machine. Although the van is stopped. The next shot shows Fred still sitting at the wheel, turning it, as if he is steering.
Continuity mistake: The gang emerge from the secret tunnel and find the movie crew dressing rooms. It shows one, then cuts back to the gang. Then it cuts back and suddenly another dressing room trailer appears next to the one seen moments before. (00:17:20)
What a Night for a Knight - S1-E1
Plot hole: It seems unlikely that: 1. The curator, who's short and chubby could fit into that suit of armor. 2. How he could be strong enough to lift up the table Scooby and Shaggy are hiding under at one point, 3. That the museum has no security guards who might catch the curator and his thugs during their art forgery scheme and 4. There are eyes shown moving in the suit of armor when the gang's talking to the curator despite the fact he's the Black Knight Ghost.
Scooby-Doo and a Mummy Too! - S1-E12
Plot hole: If the guilty man knew the diamond scarab was in the hippo statue then there's no reason he couldn't have broken the statue open and taken it.
Continuity mistake: The Snow Ghost's mask goes from being a hood type to just covering the guy's face in one shot when he's stuck in the snow.
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
Revealing mistake: When Scooby and Shaggy are on the bicycle with the Green Ghosts, the Ghosts' chains are missing.
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."
A Night of Fright is No Delight - S1-E16
Revealing mistake: When Scooby, Shaggy and Velma are on the drainpipe, it breaks and throws them into a cave. But the drainpipe hits the ocean like it is glass.
Other mistake: When the gang are swimming, a plant in the foreground moves along with them.
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)
Continuity mistake: Shaggy and Scooby stop to play a game of pool. When Scooby looks up and notices the ape head trophy has vanished, he has 4 of the balls by him, including the 6 ball. It cuts over to Shaggy, and he has the exact same set of balls next to him. (00:14:50)
Continuity mistake: Scooby finds a half eaten hamburger in a plate on the floor. When the camera cuts further back to show Shaggy, the plate has moved on the floor, as noted by the lines between boards and the placement of the plate and the spider web on the wall. (00:07:20)
Continuity mistake: When Shaggy makes the joke about living without a smart Ape Man, he's lying on his stomach with his legs in the air behind him. His feet are apart as he relaxes. Then suddenly in the same shot, they just jump close together and then slowly spread back apart. (00:06:50)
Continuity mistake: After Candy and the stunt man quit the movie, Mr. Maxwell says he has to withdraw his job offer to the gang as it's too dangerous. As the gang is shown, Scooby is sitting in front of Shaggy and Fred. It cuts to a close up of him as he gets excited about this, and suddenly Fred and Shaggy have vanished. (00:06:10)
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)
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.