Revealing mistake: When John picks his wife up at the airport, it's raining. But look at the people around outside, they are in T-shirts, in the sun, and not getting wet. Obviously the rain is being sprayed on the car.
Revealing mistake: After Mitchie wakes up at the start, her hand slides across the keyboard keys in the close-up, but that hand is not Demi Lovato's. The older looking hand has protruding veins and the fingernails are long, whereas Mitchie's nails are short and her hands look as young as she does.
Revealing mistake: When Budderball is rolling down the hill in a snowball, he hits a tree and falls out. You can tell it is just a stuffed animal since there is a stitch on the top of his head.
Revealing mistake: When Jared is first reading the field guide, in flashback, there is a shot of Mulgarath in crow form trying to take the book from Arthur Spiderwick. Later in the movie, when Jared is about to burn the book, the same shot of Mulgarath is used again.
Revealing mistake: Throughout the racing shots of the movie, the close up shots of the fans in the stands show the same people used multiple times as evident by them wearing the same clothes, poses, etc.
Revealing mistake: After Trevor has been lowered above the pit of magma, in an attempt to light the magnesium walls with a flare, we see shots of Sean hanging over the edge of the inside of the T-Rex skull. Sean's hands are resting on a large tooth in the skull, which clearly shakes like a fake prop each time he touches it. (01:17:15)