Factual error: At the start of the movie it said "NEW YORK CITY 2000", then went to a panoramic view of New York City minus the Twin Towers. Even though they stated the year was 2000 and they didn't go down until 2001.
Character mistake: In the scene when Dave is checking out the failed transmitter at the radio station, Dave says "the bad news is your return loss - way too high". This is a mistake, as the movie makers got confused between the terms "return loss" and "insertion loss". Dave should have correctly said either "your return loss - way too low" or "your insertion loss - way too high".
Continuity mistake: In the scene when Balthazar and Dave are in Chinatown looking for the Grimhold, Balthazar enters a building to talk to its current owner and encounters Horvath. As Horvath is telling Balthazar that he has released the Chinese sorcerer Sun-lok from the Grimhold, we see him standing behind Balthazar, preparing to attack him with hundreds of accupuncture needles. We see all of the needles float up and out of a doll-like figure on a table as well as some other places. Immediately after the attack, Balthazar runs across the room to the window and the doll is visible with all of the needles back in place.
Continuity mistake: When Bennett gives David Stutler the red handkerchief David has no ring. When David opens the handkerchief he is wearing the dragon ring in the shot and then it disappears in the next shot.
Revealing mistake: Horvath is trapped in the bathroom mirror, and after knocking on the mirror from the inside, a boy comes over. You can see that the shadow from the boy's shoulder is falling on Horvath, and when he turns and falls over, if you watch closely, you can see that the boy bumps into Horvath's arm. This is because the boy actor is standing in front of Horvath's actor to achieve the mirror effect.
Continuity mistake: In the scene when Balthazar changes the Rolls Royce Phantom (left seated driver) into a Mercedes McLaren (right seated driver), he is sitting in the passenger seat and reaches outside the left door window before the vehicle changes into a left seat for the driver.Can be confirmed on the youtube clip at: http://www.youtube.com/watch?v=C3DAmaMF3TQ.