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 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: In the basement as Chris irons Father Karras' clean shirt, her cup on the ironing table rotates 45 degrees throughout the scene without being touched. (01:25:55)
Deliberate mistake: Det. Kinderman appears between shots sitting on the bleachers as Father Karras is out for his early morning jog around the track. (01:03:00)
Continuity mistake: When Det. Kinderman meets Father Karras by the bleachers, Damien's collar is up. In the next shot it isn't. (01:04:00)
Continuity mistake: After the second set of tests on Regan, Dr. Klein asks Chris, "Do you keep any drugs in your house?". If you look closely he faces the wrong direction when he says this as Chris has not move yet from the spot where she's standing. She moves in the next shot. (00:56:35)
Continuity mistake: In his room, Father Dyer starts to sit next to Father Karras as he tries to console him. In the next shot he's standing in front of him. (00:45:45)
Continuity mistake: As he lays down, Father Dyer takes Father Karras' cigarette from him and puts it in his mouth. In the next shot it's back in Father Karras' right hand. (00:46:00)
Continuity mistake: After her bath as she gets ready for sleep Regan asks Chris, "What's wrong with me?" and her left arm goes from being under the pillow to under her head between shots. (00:43:50)
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: 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: 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: 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: 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: After she gets up from the table, Chris walks into the kitchen and we see an oil and vinegar tray on the lower level of the shelf above the cutting board. We see this spot before behind Det. Kinderman as he looks at his coffee, and the oil and vinegar tray are not there. (01:16:00)
Continuity mistake: Chris walks with Damien after they meet on the bridge. As they talk she's smoking a cigarette, which she throws down and puts out with her foot. After she says, "How you go about getting an exorcism?" the cigarette is back in her right hand. (01:21:20)
Continuity mistake: As Chris walks around the attic to see what's making the noise more candle wax appears dripping down its side between shots. (00:30:00)
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: 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)
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.