Plot hole: At the end of the movie, the witch's secretary returns Luke's pet mice to him even though she had no knowledge of their whereabouts or the fact that those two specific mice were his.
Suggested correction: Just before Luke and Helga leave the hotel, Helga gives Luke a tag with the address of where they live written on it. Luke took the tag and delivered it to the room where Miss Irvine was staying in. Miss Irvine is looking at the address as Luke and Helga leave the hotel. She most likely saw William and Mary at some point in the hotel and is clever enough to figure out that they're Luke's pets.