Revealing mistake: In the opening sequence, when the robbers are on the zip line, two Chicago Police squad cars (white cars with blue markings), not Gotham PD (blue with white markings) can be seen blocking off Van Buren Street for the filming. (00:01:05)
Revealing mistake: When the Telmaries are building their war machines, there is a shot of a two-man saw being used to cut timber. There is no sawdust falling from either the saw blade or the wood itself. (00:46:30)
Revealing mistake: When Liz, Abe and the Agent are shooting at the Tooth Fairies, the Agent is holding his gun in his right hand, and you can see the fingers of his left hand holding on to the top of the gun's slide while he is firing, showing that it's a prop gun and the slide isn't moving.
Revealing mistake: During the song "Mark it Up", when Pavi supposedly snatches the panties/thongs off of the two girls, you can see they are already in his hands before he does so.
Revealing mistake: During the scene when Mary is in the hospital and she flatlines, if you look closely at the screen of the monitor the reading says "leads off" meaning that the cardiac monitoring wires are unhooked from her body.
Revealing mistake: In the auditorium, when Tea Leoni is giving her speech about Pepe the mummy, Greg Kinnear (who is not visible to anyone but Ricky Gervais) starts yelling and Ricky Gervais gets up to walk away. As Greg Kinnear is yelling, an extra a few rows up turns to look at him. That shouldn't happen because the whole plot hinges on the fact that no one can see or hear these people besides Ricky Gervais.
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: 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)
Revealing mistake: When Cohen complains about how nobody takes him seriously because he doesn't have teeth anymore, you can see a full set of teeth in his mouth.
Revealing mistake: Throughout the Demolition Derby, most of the people in the audience are drawn without faces.