Other mistake: Before the Grand High Witch starts the meeting, Luke is watching the witches sit down, and he doesn't recognize the Woman in Black. This doesn't make sense because the Woman in Black attempted to kill him during the treehouse scene before any of them arrived at the hotel. The Woman in Black is even the first witch who tries to capture him when he runs on the stage.
Continuity mistake: When the evil witches start turning into mice, there is a close-up shot of the Woman in Black with mouse ears and whiskers, and her wig has fallen off. When the Grand High Witch is holding the spoon, there is a background shot of her dining room table, and the Woman in Black's wig is back on her head.
Continuity mistake: When Grandma says "Bruno's dad is about to have the soup", her right hand appears/disappears holding the bag depending on the shot.