Continuity mistake: As the Language Lab Director plays the tape of Regan backwards for Damien he turns on the wrong tape recorder. There's no indication that this is not the first time he's heard the tape and he uses the recorder on the left to listen, and is not dubbing anything onto the recorder on the right. When we hear the backwards tape he uses the recorder on the right which does not have the right tape spooled up and doesn't have an overdub of the original to play backwards either. (01:35:40)
Continuity mistake: The Language Lab Director plays the tape of Regan backwards for Damien but if you look at the tape it's running in the same direction. (01:35:45)
Continuity mistake: Back at his apartment Damien plays the tape of Regan backwards again on his tape recorder. Some items on the 1st and 2nd shelf on the back wall are rearranged between shots. (01:35:55)
Continuity mistake: In his apartment as Father Karras listens to the backwards tape of the voices coming from Regan, the tape recorder moves about four inches away from the edge of the desk between shots. (01:36:20)
Continuity mistake: Father Karras goes to see Bishop Michael trying to convince him that an Exorcism would be appropriate for Regan. If you look closely the chair on Father Karras' left, the chord to the black phone and the waste paper basket all move between shots. (01:38:35)
Continuity mistake: Father Merrin prays with his Rosary Beads as Chris brings him a cup of coffee. At first the cross is up near his hand but in the next shot it's dangling close to the floor. (01:41:55)
Continuity mistake: Father Merrin opens his red prayer book on the bedspread on the edge of Regan's bed and the page marker can be seen as he opens it. In subsequent shots the marker is not in his book, but can be seen in Damien's. (01:45:00)
Continuity mistake: Father Merrin says the Lord's Prayer as he stands at the edge of Regan's bed. At first she's sticking her tongue out at him then in the next shot she's not. (01:45:20)
Continuity mistake: Father Merrin says, "Hear my prayer" as he puts his hand on Regan's head. If you look closely the vomit first comes out when her head is facing the right and it totally soils her blue nightgown. In the next shot with her head facing left there's nothing on her blue nightgown. (01:48:05)
Continuity mistake: Father Merrin says, "I cast you out", and the green vomit that was around her mouth has now disappeared. (01:49:05)
Deliberate mistake: Father Merrin says, "It is He who commands you", and we hear and see the ceiling crack above his head. In the next shot we see the ceiling above him and they are no cracks in it, but four shots later we see the exact same spot and now cracks appear even though we hear no more cracking over-dubs after the first one. (01:49:15)
Continuity mistake: Father Merrin says, "Be gone" as he walks over to the other side of Regan's bed, and the lamp on her right almost falls over. In the next shot it's not moving at all. (01:49:25)
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: As Regan spins her head around her blue nightgown is now totally clean. (01:49:50)
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: After the first part of the exorcism, Father Merrin leaves Regan's room and we see his red prayer book and crucifix sitting on the nightstand as he does. He sits on the steps and then goes into the bathroom to take a pill, but later as he goes back into her room he has the book and crucifix in his hand. (01:53:00)
Continuity mistake: After the first part of the exorcism Father Karras sits on the steps as Father Merrin walks to the bathroom. His coat and purple scarf are draped across the railing but two shots later as Father Karras goes back into Reagan's room the items have disappeared. (01:54:45)
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)
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: 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)
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