Continuity mistake: At the party, a drunk Burke Dennings calls Karl a, " Bloody, damned, butchering, Nazi pig!" and the chain around his eyeglasses moves on his chest twice between shots. (00:40:45)
Continuity mistake: Sitting in front of the fireplace at her party, Father Dyer tells Chris that Father Karras' mother had died the previous night. The candle in the candleholder to her right rotates throughout the scene. (00:40:35)
Continuity mistake: Chris takes Regan to Doctor Klein for some tests to see what's wrong with her. As she sits in the office waiting for the doctor to come in, the papers on his desk move between shots without anyone going near them. (00:34:10)
Continuity mistake: As Chris sits in the office waiting for Doctor Klein to come in, the tissue coming out of the Kleenex box behind his desk moves between shots without anyone going near it. (00:34:15)
Continuity mistake: As Damien walks into the psychiatric ward to visit his mother the woman directly across from her changes sides on her bed between shots. (00:38:15)
Continuity mistake: In Doctor Klein's office the handle on the ice bucket in the back of the room under the blue picture is moved as the scene progresses and Chris talks to the doctor. (00:33:45 - 00:35:55)
Continuity mistake: After Chris is startled by the exploding flame in the attic there's a shot of a priest caring flowers into the courtyard of a church and if you look closely the flowers on the right are the same in the close-up but the flowers on the left are not. When he walks into the chapel the flowers are rearranged again. (00:31:20)
Continuity mistake: Before Chris goes up into the attic to see what's making the noise Regan is wearing a blue nightgown. A few seconds later they cut back to her and she's wearing a pink nightgown. (00:28:30)
Continuity mistake: In bed Regan reads "Photoplay" magazine as Chris touches the covers of the bed. In the next shot she's pulling the magazine away from Regan. (00:23:30)
Continuity mistake: As Chris goes up into the attic to see what's making the noise, the flame on the candle gets longer between shots as she walks up the steps. (00:29:45)
Continuity mistake: Father Damien and Tom have a drink at a local bar where Damien confides that he has lost his faith. When he brings the beers over both glasses are full but several seconds later when he takes his first sip the head is already gone. (00:26:50)
Continuity mistake: After Chris is startled by the exploding flame in the attic there's a shot of a priest carrying flowers across a courtyard into a chapel. When he first opens the door to the chapel he's in the shade, but when we see him walking into the chapel from the other side of the door the sun is shining. (00:31:30)
Continuity mistake: Chris talks to Regan as she puts her to bed. If you look closely the large pictures on her night table change position throughout the scene. (00:23:45)
Continuity mistake: In the scene in their basement Chris brings the Ouija board over to the ping pong table and Regan puts the paddle down with the ball underneath. In the next shot the paddle is there but the ball disappears. (00:22:45)
Continuity mistake: As Father Karras goes to New York to see his mother a woman in yellow on the sidewalk with her two children disappear between shots as he crosses the street. (00:18:30)
Continuity mistake: As Chris and Regan discuss how to play the Ouija board the pointer is to the right of the word No. In the next shot when Regan picks it up it's covering the word No. (00:23:10)
Continuity mistake: In the scene in their basement Chris takes the painted bird from Regan who touches the wood post in the center of the room with her right hand. In the next shot she's leaning against it. (00:22:15)
Continuity mistake: After returning home from the movie set Chris walks in and says hello to Sharon. The bottle of Coke, the typewriter carriage and the page she is typing all move between shots by the time Chris gets to the desk. (00:16:10)
Continuity mistake: After returning home from the movie set, Chris gets a letter from the White House which she reads with both hands. Regan says, "Hi Mom" and in the next shot Chris has a drink in her hand. (00:16:35)
Continuity mistake: At the movie set, Chris says to Burke Dennings, "It just doesn't make sense" and the guy in the blue shirt in the background is standing up. In the next shot he's bending over. (00:13:20)
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