Plot hole: The spell Magicae Maxima works, but it shouldn't have worked at all because there was one ingredient missing, the petrified spider.
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.
Factual error: In one scene, Winifred asks Gilbert who Trask is and Gilbert replies that Trask is the "mayor", to which Winifred just acknowledges. However, the office of mayor was not created in Massachusetts until 1823, which is 130 years after they were hanged, so given the witch's lack of knowledge of the modern day, she would have likely not known what Gilbert meant.
