Revealing mistake: When Shaggy, Scooby and Mary Jane have escaped the monsters using the quad bikes, they land in a small pit and Scooby starts barking at Mary Jane. When Shaggy says, "Scooby-Doo like what are you doin' man", Shaggy is talking to the floor - he's not looking anywhere near the CGI Scooby.
Revealing mistake: When Daphne and the wrestler are fighting on the roof, Daphne slams the door into the wrestler and the wrestler's head smashes through the door window. The wrestler then grabs Daphne's arm, and smashes his arm sideways into the doorframe. If you look closely, you may have to watch a few times, you can clearly see the pre-cut line that makes the doorframe easier for him to smash.