Goofs

(at around 1h 40 mins, extended version only) When Father Merrin asks Chris MacNeil what Regan's middle name is, she is wearing a fur coat. When the priests enter Regan's room and Father Karras closes the door in her face, Mrs. MacNeil is no longer wearing the coat.

(at around 41 mins) When Father Karras is in the dorm sitting on the bed smoking a cigarette, the priest takes the cigarette from Karras and lays him down on the bed. When Father Karras sits up and grabs the priest's arm, the cigarette is still in Karras's right hand. In the next shot, the cigarette is back with the other priest.

(at around 10 mins) In the establishing shot of Georgetown, a building's signs read "Merry Christmas" and "Happy New Year." Yet not much later in the film, Chris walks past some trick-or-treating kids.

(at around 1h 16 mins) The cigarette that Chris stomps out in the park is back in her hand a moment later.

(at around 1h 45 mins) At the beginning of the exorcism, Regan's blanket is yellow. Towards the end, when Father Karras is checking her heart, the blanket is blue.

(at around 55 mins) Despite Dennings being killed the police do not close off the roads or even investigate the MacNeil household. If a person is found dead outside a house or other building like in the film the police would have to search the place for clues.

(at around 46 mins) The first medical test Regan has is an arteriogram. This test does require contrast dye to be injected into a large artery, but it is usually the femoral artery (groin) or brachial artery (arm). Very rarely would the carotid artery in the neck be used. The femoral artery is most often used based on its easy access.

(at around 1 min) The activity at the archaeology dig is energetic beyond all reason. No responsible archaeologist would risk the extreme amount of damage that such activity would produce.

(at around 1h 40 mins) Steaming hot water comes out of the tap as quickly as Damien turns on the faucet. It's not possible for water that hot to come through plumbing that fast, especially since the bedroom was on the highest floor of the house and the temperature of the room was at least freezing.

(at around 1h 24 mins) When Father Karras is saying the words of consecration during Mass, he says Jesus "took the bread, broke it, (etc.)," then he breaks the host. In a real Catholic Mass, the priest does not actually break the bread at that point, even though he says those words.

(at around 1h 29 mins) When Karras listens to the recording of the possessed Regan backwards, the questions the priest asks her are heard speaking forward. The questions should be heard backwards.

Correction: Karras isn't heard at all when he's listening to the tapes. Only Regan is heard speaking backwards at first, and then forwards after the linguist explains she is speaking plain English in reverse. Karras's voice is not heard at all in that sequence, only Regan's.

Near the end of the picture, Father Karras's stole seems to disappear in Regan's room, but that's because he puts a blue pullover on top of it because it's cold in there.

(at around 1h 23 mins) When Karras first leaves the MacNeil household Kinderman is seen looking from his car at a person behind the curtains which is implied to be Regan despite the fact she is strapped to her bed. That however is the point of the scene: That Kinderman sees what should not be possible: Regan moving freely about despite being ill and strapped to the bed.

(at around 1h 40 mins) During the exorcism, the plasma bottle and stand are tipped over, and the nearly-full bottle smashes on the floor. No liquid comes out. That's because the plasma has frozen solid inside the bottle. Looking closely, it can be seen that the plasma shatters along with the glass.

No explanation as to how Regan's head can spin around without her neck being broken and killing her.

Correction: After much debate over the head turning 360 degrees around, director William Friedkin compromised with author William Peter Blatty by adding a shot of Father Karras, implying it's a hallucination being projected by the demon.

(at around 1h 40 mins) During the exorcism, Regan is lying on the bed with her head to one side while green vomit is pouring from her mouth onto the bed. When she turns her head to the other side, the tube that pumps this "vomit" is visible on the back of her cheek, just under her ear, which in turn is shown devoid of make-up after the pressure from the pump tube lifts the prosthetic mask away from her neck.

(at around 1h 20 mins) When Regan vomits on Father Karras the first time he meets her, it can be seen that a stunt double is doing the vomiting.

(at around 30 mins) When Chris first checks the attic and her candle shoots a flame, a small tube is briefly visible attached to the base of the candlestick which then goes under the sleeve of the actress, possibly the gas tube that makes the effect of the changing flame.

(at around 1h 40 mins) Wires holding Regan can be seen during the exorcism levitation scenes.

(at around 1h 13 mins) The wire that yanks Chris (when possessed Regan smacks her to the ground) can be seen lifting the bottom of the curtain where her head lands.

(at around 1h 20 mins) In the basement scene where Father Karras visits the Georgetown home rented by Chris, the 2x10 ceiling joists are fairly new looking despite the fact that it is a Federal-style home (c. 1780-1830) revealing it as a built set and not an actual location of that era.

(at around 1h 35 mins) When Regan insults Karras's mother, the lines don't match her mouth movements.

(at around 58 mins) When Karras is jogging round the sports field he stops when Kinderman addresses him throughout this scene heavy breathing can be heard from Karras even though he is able to utter such lines as "Have we met?" and "What's this all about?" very coherently indicating that the puffing and panting sound from Karras was added in post production. Also his mouth never moves during the alleged exhale/inhale scenes.

(at around 57 mins) In the birds-eye view of the scene where Damien is running around the track before meeting the detective, there is a fly on the camera lens.

(at around 4 mins) Once, in the Iraq sequence, the shadow of a microphone is visible on Father Merrin's hat.

William Peter Blatty closely modeled the exorcism scene on the actual rite of exorcism in the Church's "Rituale Romanum". Father Merrin can be seen opening a copy of the Rituale in the scene in question. However, the priests depart from the Rituale in two important details. First, there should have been four people (apart from Regan) in the room during the exorcism: the exorcist himself; an assistant priest to take over in case the exorcist died midway through; a member of the victim's family of the same sex as the victim, to help restrain her; and a doctor, to (among other things) administer any medication that was needed. Due to the "2 Priest rule", Fr Merrin should have delayed the second round of the exorcism and phoned the bishop to get a replacement for Fr Karras, instead of trying to tackle it on his own.

(at around 1h 4 mins) One character says that exorcism is still practiced only in the Catholic church. That is incorrect -- it is also practiced in the Eastern Orthodox tradition, and possibly unofficially in other religions.