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: Chris and Regan walk back to Sharon's desk and the radio on the shelf to the right of the desk has moved between shots. (00:17:20)
Continuity mistake: Before Father Karras walks into his mother's apartment the lights are on. In the next shot as he walks in the lights are off and he turns them on. (00:19:10)
Continuity mistake: Father Merrin watches three metal smiths working and the piece of metal on the anvil is red hot in one shot and not in the next. (00:05:45)
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: Father Karras says to Det. Kinderman, "It didn't happen in the fall?" And his right hand goes from holding the fence to being wrapped around the poll between shots. (01:05:30)
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: When the full IV bottle falls and breaks during the exorcism, no liquid splashes out of it.
Other mistake: When Karras throws himself out of the window, why is a man who needs a cane running full speed down those steep steps? It makes no sense since people use canes for balance.
Suggested correction: Some people have a cane as a fashion statement. Also, it's possible the cane helps but is not totally necessary, certainly able to be ignored when rushing to an emergency.
Revealing mistake: When possessed, Regan slaps Chris after saying, "Lick me!" You can see a wire attached to Chris' back pulling her to the ground while hidden behind the curtain. (01:19:07)
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: Throughout the film it has been made clear that it's Autumn and it is cold. Leaves have fallen and they are crunchy on the ground. In the scene where Father Karras is walking with the detective they turn a corner and it's summer again. The sky in the scene goes from grey to blue and the trees have re-greened as they pass a tennis court with players in their short whites. (00:43:00)
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: Damien starts the tape recorder in Regan's room twice between shots. (01:31:40)
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: 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: Father Karras confronts Regan for the first time and just before she pukes on him fresh vomit appears on her night gown. Her eye makeup becomes darker and her hair is matted down also between shots. (01:25:35)
