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.
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.
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".
Revealing mistake: Horvath is trapped in the bathroom mirror, and after knocking on the mirror from the inside, a boy comes over. You can clearing 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.
Plot hole: After being released from Grimhold, Morgana is ready to cast "The Rising" spell. She never actually learned it, or even saw the parchment with the spell, since it was Horvath who took it, then left. Afterwards, Morgana is quickly trapped in Grimhold - it means she doesn't know how to cast this spell, which she actually does like a sorceress supreme.
Factual error: During the scene in Chinatown Horvath refers to Sun Lok as "an old associate" "that lived 200 years ago". Horvath was in the Grimhold 200years ago. How could he have known him?
New this month Plot hole: It is assumed that Horvath uses the same mirror spell during the car chase that Balthazar used in the bathroom. If that's the case, wouldn't Horvath have been able to bring himself out of the mirror by entering his own reflection as Balthazar and David did in the car?
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.
Other mistake: Towards the end of the movie, in the scene where Becky is seen running to the park and into the arms of Dave, it is unclear as to how exactly she got there. Are we to assume she ran all the way? One may say she took a cab, but the absence of a purse indicates that this is unlikely.
Plot hole: In the scene where Horvath tracks the Grimhold and kills the man in the car he mentions Chinatown. How would he know of the existence of Chinatown and refer to it by its common name after being trapped in the grimhold and urn for a thousand years?