Deliberate mistake: When Harry drops the Monster Book of Monsters, in the first shot facing Harry's feet as the book chomps at him, he runs backwards a few steps and he stands at the foot of the bed. However, in the shot facing the fireplace, there is actually plenty of floorspace now, much more than other shots, between the rug and bed for Harry to continue to run backwards. (00:15:15)
Deliberate mistake: Old Weird Harold's pants fall down twice in the film. His boxers are different colors each time; once they're red, the other time they're blue. The director admits to this deliberate mistake in the commentary, siting his interest in seeing if people will notice.
Deliberate mistake: In the shot where the camera zooms in on the car, after they realise they are standing on a train track, it is obvious that the car would be hit by the train even if they managed to change tracks. The car's hood is parked right above the other track. (00:23:45)
Deliberate mistake: In the first sequence, you see a man riding on the wooden battering-ram, and you can see where two different shots of the same man were spliced together, as the background suddenly shifts from being a forest, to being the exterior of the town, where the stone wall is. It was meant to be one single shot, but you can just see how the trick works. (00:00:30)