Factual error: In the hypnosis scene, when father Marrin is crossing himself, he is doing it the wrong way - from right to left. (00:20:40)
Factual error: Father Lamont tears out the Regan doppelganger's heart from where her stomach should be. Even allowing that she's a manifestation of a demon and may not conform to human anatomy, it isn't plausible that Lamont would know to grab for her heart through her abdomen.