Continuity mistake: At her front door, Father Karras asks Chris, "Did Regan know a priest was coming over?" as he looks at her. In the next shot he's looking down. (01:28:10)
Continuity mistake: As Father Karras listens to a tape recording of Regan at the university language lab we see the tape recorder running, a pen in his right hand and no ash tray or cigarette. In the next shot from behind smoke rises from a lit cigarette and the pen is on the desk. (01:29:15)
Continuity mistake: As Father Karras listens to a tape recording of Regan at the university language lab the lamp above his head is on his left. In the next shot it's right in front of him. (01:29:30)
Continuity mistake: Father Karras listens to a tape recording of Regan at the university language lab and there are three rows of seats in front of him. In the next shot from behind there are two rows in front of him. (01:29:35)
Continuity mistake: Damien brings a tape recorder to Regan's room and a brown/green plastic bowl is on the table to her left. In the next shot when the draw of the table pops out the plastic bowl is moved back about four inches. (01:31:10)
Continuity mistake: Damien starts the tape recorder in Regan's room twice between shots. (01:31:40)
Continuity mistake: Lots of stuff moves as Damien waits on the couch in Chris's living room as she pours him a drink. The flower pot on the back wall under the ceramic pumpkin moves to the left and the couch he's sitting on moves a couple of feet to the left. Later everything moves back. (01:34:00)
Continuity mistake: Damien plays the tape of Regan for the Language Lab Director and when we first see them a chair is in front of Damien. In the next shot the chair is gone and a coat appears next to him on his right. (01:35:30)
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)
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)
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