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 the gang is chased out through the window, Scooby pushes them on a cart. During the shot, on the floor, you can see the tracks that the cart is travelling on.
Revealing mistake: In the little clip at the end, Scooby Doo opens a jar of peppers and dumps them in his mouth. You can see that there should be more in the jar than the seven or so that fall into his mouth, but suddenly the jar is empty.
Revealing mistake: After the little Scooby-gang has crashed through a window after being chased by monsters they bounce upon an awning and you can see that the glass from the window is now paper.
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. (01:09:40)