When Murray casts a wish in the school corridor, the red paper and pictures on the noticeboard behind him disappear and re-appear numerous times.
As Oliver is chasing for the runaway driverless carriage, you can see a man driving the carriage.
A flail is incorrectly referred to as a mace. The difference is that a flail's striking head is linked to the weapon's handle by a chain. A mace is a rigid handle ending in a striking head.
Claudia casts a spell on Hortense that flattens her. She then tells Hortense she thinks of her as being "one-dimensional". In order for the pun to work, Hortense would have to be turned into a simple line, having only length as a dimension. As is, Hortense is reduced to two dimensions, having both length and width.