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)
Continuity mistake: At the end of the movie Chris calls to Father Dyer from her car and a Volkswagen beetle appears parked in front of them on the right side of the road, even though we've heard no car passing them. (02:05:45)
Continuity mistake: When Chris is taking a walk with the priest Damien, she throws her cigarette to the ground, but in the next shot it's back in her hand again.
Continuity mistake: After Regan's head spins around, her hairstyle around the forehead changes between one frame and another.
Continuity mistake: When Father Karras visits his mother, he places his collar edge-wise on the shelf. In the next shot the collar is lying down at a different angle.
Continuity mistake: When Regan throws up green vomit over Father Merrin's purple sash, Damien takes it and his glasses to the ensuite, to wash them, but when he gets there, the glasses are no longer with him.
Continuity mistake: In the scene where Chris McNeil goes into the attic to investigate, the size of her candle shrinks at an incredible rate between shots.
Continuity mistake: When Damian goes to visit his mother in her apartment, the bed is on the right side of the front door, but when he leaves, now, suddenly, the bed is on the left side of the front door.
Continuity mistake: When the full IV bottle falls and breaks during the exorcism, no liquid splashes out of it.
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: After the exorcism, Regan's wrists are tied together, but are then later seen tied to the bed.
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.