Revealing mistake: When the gang tries the growth formula in the theatre, they quickly grow out of all their clothes until they are naked. But the scrunchy in Jean's pony tail is still in place and shows no signs of being stretched, even if the pony tail now is 20 times bigger.
Revealing mistake: In the dance hall scene, you can see the wires attached to the ducks to make them move.
Revealing mistake: When the teens are at the theater for the first time, one of the teens on the right leans on a statue that bends after he leans. He then looks over quickly and stops leaning. He then looks sheepishly at the camera.
Revealing mistake: When the girl on the motorcycle is trying to tie Mike's legs together by driving around them in circles, his legs are obviously props. This also goes for later, when he is lying on the ground and the townspeople are trying to tie him up. When his legs are in shot, they look fake, and never moves an inch.