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: When Chris is taking a walk with the priest Damien, she throws her cigarette to the ground, but in the next shot it's back in her hand again. (01:21:30 - 01:22:06)
Continuity mistake: After Regan's head spins around, her hairstyle around the forehead changes between one frame and another.
Continuity mistake: Father Merrin says, "I cast you out", and the green vomit that was around her mouth has now disappeared. (01:49:05)
Continuity mistake: As Regan spins her head around her blue nightgown is now totally clean. (01:49:50)
Continuity mistake: Regan sits up in bed as Father Merrin says, "By this sign of the Holy Cross" and she has no frozen vapor coming out of her mouth. In the next shot before her head spins around frozen vapor appears and can be seen all the way down to her blanket. (01:49:35)
Continuity mistake: Father Dyer runs to Chris' car at the end of the movie as we see another car coming at them and passing on their left. In the next shot that car has disappeared. (02:05:25)
Continuity mistake: At the movie set, Chris yells, "Hold it, hold it" as Father Karras watches and walks away. In the next shot several extras along with the line of shade behind him disappear. (00:14:50)
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: When Father Karras visits his mother, he places his collar edge-wise on the shelf. In the next shot the collar is lying down at a different angle.
Continuity mistake: Father Karras eats dinner in his mother's apartment and the curtain on the far side of the table moves without being touched the second time we see it. (00:20:25)
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)
Continuity mistake: As the first part of the exorcism comes to a close, Father Karras picks the blanket up off the floor and covers Regan with it. Father Merrin's red prayer book can be seen on the left night table but in the next shot it's moved and we see his crucifix there instead. (01:53:00)
Continuity mistake: When Damien jumps through Regan's window we can see that half of the window frame is still there. But a couple of shots later when Det. Kinderman looks through it almost all the window is knocked out. (02:01:40)
Continuity mistake: After Chris returns home from the movie set, items on the window sill, the flowers on Sharon's desk and the pencils in her pencil holder all move inbetween the two close-up shots of her as she sits at her desk. (00:16:50)
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: After Regan walks in and says to the Astronaut, "You're gonna die up there" Chris gives her a bath. When we first see them she's sponging her down with her right hand and soapy water. In the next shot she's using her left hand and the water is clear. (00:43:20)
Continuity mistake: When we first see her, Chris MacNeil lays in bed editing her script. The lamp above her head changes position between shots after she turns it on. (00:10:50)
Continuity mistake: At the end of the movie Chris calls to Father Dyer from her car and a Volkswagen beetle appears parked in front of them on the right side of the road, even though we've heard no car passing them. (02:05:45)
Continuity mistake: Damien goes back into Reagan's room and takes the towel on the night stand to her right and wipes her forehead with it. He never puts it back on the nightstand but when Father Merrin comes back into the room it's there again. (01:56:15)
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