Continuity: When Jared and Mallory are running through the tunnels, Mallory's eyeglass disappears in one shot.

Audio problem: When Jared is arguing with his mom in the car, his mouth isn't moving.

Revealing: 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.