Factual error: When Mike first starts listening to the session tapes, the camera does a slow pan down the label on the box. Quite clearly, we can see that the patient's diagnosis is listed as "D.I.D.", along with a few names. Since the hospital closed in the mid 80's, it is not possible for her to have that diagnosis. Before the early 1990's, D.I.D. (Dissociative Identity Disorder) was known as M.P.D. (Multiple Personality Disorder). In later scenes it is correctly listed in the woman's files as Multiple Personality.
Continuity mistake: When Gordon is remembering the day he went home and his wife spilled the hot water on him, it shows Gordon in the van watching his wife and daughter walk into the house. They disappear around the corner, but in the next shot of the house his wife and daughter are walking into the house again.