Plot hole: The spell Magicae Maxima works, but it shouldn't have worked at all because there was one ingredient missing, the petrified spider.
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Suggested correction: The spell works because Winifred believes it is a petrified spider. It also works because she believes that Billy Butcherson was her lover; which he wasn't, as stated several times in the movie: in the flashback scene, Billy mentions it several times to Gilbert, and especially when Sarah tells Winifred that Billy was her (Sarah's) lover, to which Winifred replies that Sarah was just a fling; so the spell can work with incorrect material as long as Winifred, as the main witch, believes it.