When Freddy takes the map, he's looking at the back until Scooby's close-up, when the map is suddenly turned correctly.
When the gang finds the door that fits the key, the room looks different in two different shots.
In the first close-up of the gang on the boat, Velma's skirt loses its pleats the entire time. At the same time, Scooby's spots on his back are missing, even when the camera switches to another close-up showing just him and Velma.
The page they're on says "2", despite the fact that they're halfway through the book.
When Shaggy says, "Groovy," his eyes disappear.
The rims on Daphne's cuffs are whitish on close-up of her holding the key.
When Fred says, "This is the place!", Daphne's pantyhose are missing.
During Velma's scientific explanation of fog, she calls it "mugilation" which is not an actual word.