Continuity mistake: After the second set of tests on Regan, Dr. Klein asks Chris, "Do you keep any drugs in your house?". If you look closely he faces the wrong direction when he says this as Chris has not move yet from the spot where she's standing. She moves in the next shot. (00:56:35)
Continuity mistake: As they speak about Regan, Chris sits down at his desk and a shadow appears on Dr. Klein's forehead that wasn't there in the previous shot before she walked to the seat. (00:56:55)
Continuity mistake: Chris drives home past the police cars and ambulance outside her house after Burke Dennings is killed on the steps. At first there are a bunch of people on the steps, then the next time we see it two seconds later there's a lone policeman standing there. (00:57:45)
Continuity mistake: Chris drives home past the police cars and ambulance out side her house and when she gets in there's no one there. If you look closely as she turns into her driveway, tire marks on the wet pavement can be seen in the exact same spot from a previous take. (00:57:50)
Continuity mistake: After Burke Dennings is killed, Chris drives home past the police cars and ambulance out side her house with her high beams on. In the next shot as she pulls into her driveway, her low beams are on. (00:57:55)
Continuity mistake: After Burke Dennings is killed, Chris goes upstairs and finds the window in Regan's room is wide open. It's so cold that you can see the condensation from her breath in some shots, but not in others. (00:59:10)
Continuity mistake: Chuck, the Assistant Director comes in and tells Chris and Sharon that Burke Dennings has been killed. Chris turns around and says, "Heard what" and in a bad over-dub we see her mouth move as she says something else that is been edited out. (00:59:50)
Continuity mistake: Chuck, the Assistant Director leaves after telling Chris and Sharon that Burke Dennings has been killed. In the next shot Chris' scarf is tied differently around the back of her neck. (01:00:30)
Continuity mistake: The long curtain on the window behind her has been noticeably moved between shots as a doctor puts Regan in a trance. (01:01:50)
Continuity mistake: The Psychiatrist hypnotizes Regan and the blanket on the chair behind her head is rearranged several times between shots. (01:01:50)
Continuity mistake: The Psychiatrist that hypnotizes Regan is wearing a tie that in the close-ups, is retied differently as the scene progresses. (01:02:05)
Continuity mistake: Father Karras is out for an early morning jog and jogs past the same place twice as he jumps three-quarters of the track between shots. (01:02:50)
Deliberate mistake: Det. Kinderman appears between shots sitting on the bleachers as Father Karras is out for his early morning jog around the track. (01:03:00)
Continuity mistake: When Det. Kinderman meets Father Karras by the bleachers, Damien's collar is up. In the next shot it isn't. (01:04:00)
Continuity mistake: As they walk, Det. Kinderman talks about Burke Dennings death and Father Karras says, " Only what I read in the papers". Father Karras goes from not holding the towel around his neck to holding it between shots. (01:04:15)
Continuity mistake: As they talk, Det. Kinderman describes how Burke Dennings was found, "with his head turned completely around, facing backwards". In the next shot they're standing in a different place. At first Father Karras has a shiny fence clamp on the right of his head than in the next shot it disappears. (01:05:20)
Continuity mistake: Father Karras says to Det. Kinderman, "It didn't happen in the fall?" And his right hand goes from holding the fence to being wrapped around the poll between shots. (01:05:30)
Continuity mistake: As Father Karras talks to Det. Kinderman by the tennis court fence, the towel around his neck goes from being longer on the right side to longer on the left between shots. (01:05:40)
Deliberate mistake: Det. Kinderman walks up the stairs where Burke Dennings fell to his death and looks up at Regan's window. This could not possibly be the outside of her window because when we see the room from the inside as Chris closes the window near the beginning of the movie, there's at least 6 to 8 feet between the window and corner of the house. Now when we see it as he looks up, the window is right at the corner of the house. (01:12:35)
Continuity mistake: Chris interrogates Sharon and Willi about who put the cross under Regan's pillow, and Sharon goes from touching the phone with her left hand to holding a pad of paper between shots. (01:12:45)
Answer: The demon wants everyone to lose hope for the girl and give up on her. Her father, having already flown the coop, just makes this goal that much easier.
TonyPH