Audio problem: At the end of the movie Chris calls to Father Dyer from her car and says, "I thought you'd like to keep this". If you read her lips she just says, "I thought you'd like this". (02:05:25)
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)
Revealing mistake: You can see wires in the scene where Regan levitates above her bed in the exorcism scene, lifting her up. The way they pull on her dress is also visible.
Visible crew/equipment: Near the beginning of the movie, the old priest is in Northern Iraq and you can see the shadow of a microphone on his hat.
Revealing mistake: Crash mats are visible on the steps, when the Father is thrown through the window and rolls down the stairs.
Continuity mistake: After Regan's head spins around, her hairstyle around the forehead changes between one frame and another.
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.
Revealing mistake: As Father Merrin puts his hand on Regan’s head, nasty green vomit starts to come out of her mouth. If you look closely when she turns her head towards him you can see the hose under her ear that proceeds under a false cheek and into her mouth.
Visible crew/equipment: When the Fathers are exorcising Regan, as she floats above her bed, the boom is visible at the extreme top of the screen.
Revealing mistake: In some extended edition cuts of the film, Regan backwards spider walks down a flight of stairs, and blood falls from her mouth. But her face isn't that of Linda Blair, but of a stunt double.
Revealing mistake: When Regan meets Father Karras, she projectile vomits in his face. But if you look closely while this vomit happens, you can very easily notice that it is the stunt double.
Continuity mistake: When Regan throws up green vomit over Father Merrin's purple sash, Damien takes it and his glasses to the ensuite, to wash them, but when he gets there, the glasses are no longer with him.
Continuity mistake: In the scene where Chris McNeil goes into the attic to investigate, the size of her candle shrinks at an incredible rate between shots.
Continuity mistake: When Damian goes to visit his mother in her apartment, the bed is on the right side of the front door, but when he leaves, now, suddenly, the bed is on the left side of the front door.
Continuity mistake: When the full IV bottle falls and breaks during the exorcism, no liquid splashes out of it.
Continuity mistake: When Kinderman introduces himself to Karras at the track, he puts his ID in a pocket in his vest. The rest of the time they're talking and walking, he must be wearing a different vest because there's no pocket.
Continuity mistake: When Regan is in the hospital, to calm her down the doctor gives her an injection in the arm. In one shot her gown is up high on her shoulder, but in the next the gown is down around her upper arm - the doctor can hardly find room to inject her it's so low.
Continuity mistake: During the exorcism, the first time, the wardrobe door is cracked, but later it is solid.
Continuity mistake: The first time Father Merrin enters Regan's room, Chris is wearing a fur coat, because Regan's room is so cold, but when Damien closes the door, Chris's fur coat is gone.
Continuity mistake: After the exorcism, Regan's wrists are tied together, but are then later seen tied to the bed.
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