Continuity mistake: Towards the end of the film the amount of monkshood inside Brigette's needle changes several times.
Plot hole: The scene where the sisters are talking about killing the popular girl in class during P.E. during a game of Lacrosse, Bridgette is pushed into the body of a disemboweled dog. Everyone then starts to freak out seeing the dead animal. How could anyone, the students, teachers, grounds-keeper, not see that corpse until Bridgette was knocked into firstname.lastname@example.org