Continuity mistake: After returning home from the movie set, Chris' necklace is tucked under her collar. She doesn't go near it but two shots later it's over her collar. (00:16:30)
Continuity mistake: Chris picks up a newspaper as she tells Carl, "We've got rats in the attic". In the next shot the newspaper is unmoved. (00:12:40)
Continuity mistake: The phone on the right of the archaeologist's desk moves between shots in the close-up. The first time we see it the connector chord is flush with the desk but the second time we see it the chord is above the desk and no one goes near it. (00:06:45)
Continuity mistake: Father Merrin walks over to see what the other archaeologist has found at the dig site and the items in the sifter move between shots in the close-up. (00:03:15)
Continuity mistake: At the dig site in the beginning of the movie, the items on the rock behind Father Merrin move in the close-up after the other archaeologist shows him what he has found. (00:03:25)
Continuity mistake: After the exorcism, Regan's wrists are tied together, but are then later seen tied to the bed.
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 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: 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: Before she gets up from the table, Chris asks Det. Kinderman, "Would you like some more coffee?". His cup is obviously empty as she picks it up but if you look closely throughout this continuous scene, he's never touched it. (01:15:50)
Continuity mistake: Chris interrogates Sharon and Willi about who put the cross under Regan's pillow, and Sharon goes from touching the phone with her left hand to holding a pad of paper between shots. (01:12:45)
Continuity mistake: As they talk, Det. Kinderman describes how Burke Dennings was found, "with his head turned completely around, facing backwards". In the next shot they're standing in a different place. At first Father Karras has a shiny fence clamp on the right of his head than in the next shot it disappears. (01:05:20)
Continuity mistake: After Chris puts her to sleep and walks down the steps, Regan screams and there is a close-up of the lamp flickering on the wall. In the next shot the lighting has changed as a dark shadow appears behind the lamp. (00:44:25)
Continuity mistake: Sitting in front of the fireplace at her party, Chris asks Father Dyer, "Who's the priest I keep seeing there " and the woman to his left has a drink in her hand. In the next shot the drink is on the table. (00:40:20)
Continuity mistake: Father Karras bandages his mother's leg and we see her right hand as she raises it and he says, "Mama, I could take you somewhere ". In the next shot she raises her hand to her head and she now has a band-aid on her wrist. (00:20:50)
Continuity mistake: In the archaeologists office at the beginning of the movie Father Merrin looks at the clock on the wall as it stops. The first time we see it the pendulum is to the left of center but the second time we see it, it's dead center and it hasn't been touched. (00:06:45)
Continuity mistake: Chris says to Carl, "We've got rats in the attic" and he replies, "Rats?". All the items on the table to her left have been rearranged between shots as she turns towards him. (00:12:40)
Continuity mistake: Father Dyer tells Chris about Father Karras' mother's death and gets the story wrong. He tells her that she died in the apartment and was there for several days before they found her. This contradicts all we saw in the previous scene in the psychiatric ward.
Continuity mistake: As they speak about Regan, Chris sits down at his desk and a shadow appears on Dr. Klein's forehead that wasn't there in the previous shot before she walked to the seat. (00:56:55)
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