Revealing mistake: In the hotel meeting scene, the Grand High Witch tells the witches they can take their wigs off. When we see the newly bald witches, it is particularly obvious that only the foreground witches are ladies with bald caps - the rest of the witches are bald men. (00:30:10 - 00:31:00)
Other mistake: When the evil witches get their just desserts, Lois Leffour's friend Julia Whitman is choking on the Formula 86 bottle. Even though the bottle is small, Julia should have either seen it on the spoon, or felt it when she put it in her mouth. Instead, Julia swallows the bottle and coughs it up during her transformation into a mouse.