The film is supposed to take place on Lake Erie but during the beach scene with Velma and Shaggy it is obvious that they are on a California beach and not Lake Erie. There are no rocky formations on the Lake Erie coastline.
(at around 1h 20 mins) When Shaggy shows the text on the back of the photo book in the mirror, instead of the text being reflected backwards, it's reflected forwards (as if it wasn't mirrored at all). This may be an intentional goof as the "mirrored" text is an important plot point.
In the shot where Scooby is mad at shaggy for dating Velma, and they are in the room, they are both standing in front of a mirror. But you can only see Shaggy's reflection in the mirror and not Scooby's.
It was the phone book on the back page it shows Trowburg's when Shaggy first shows the book in the mirror you can see it spelled backwards with the letters backwards and the 's at the beginning. The second time you see the book it spelled Grubwort's with the 's at the end of word which would make it S'Trowburg instead of Trowburg's but the rest of the page is correctly backwards.