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: 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)
Visible crew/equipment: After he stirs it, Det. Kinderman lifts the spoon out of his coffee as he starts to talk to Chris at the dinner table. A perfect reflection of the cameraman can be seen in the left of the spoon. (01:13:00)
Continuity mistake: After Chris returns home from the movie set, items on the window sill, the flowers on Sharon's desk and the pencils in her pencil holder all move inbetween the two close-up shots of her as she sits at her desk. (00:16:50)
Continuity mistake: After Burke Dennings is killed, Chris goes upstairs and finds the window in Regan's room is wide open. It's so cold that you can see the condensation from her breath in some shots, but not in others. (00:59:10)
Continuity mistake: After Regan walks in and says to the Astronaut, "You're gonna die up there" Chris gives her a bath. When we first see them she's sponging her down with her right hand and soapy water. In the next shot she's using her left hand and the water is clear. (00:43:20)
Continuity mistake: When we first see her, Chris MacNeil lays in bed editing her script. The lamp above her head changes position between shots after she turns it on. (00:10:50)
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)
Revealing mistake: When Father Karras sprinkles holy water on Regan, they forgot to paint her ear lobes and it looks terrible. (01:32:30)
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)
Visible crew/equipment: When the Fathers are exorcising Regan, as she floats above her bed, the boom is visible at the extreme top of the screen.
Revealing mistake: As Chris walks around the attic to see what's making the noise the shadow of the candlestick appears on her face which is not possible as it's the only light source up there. (00:30:15)
Audio problem: At the end of the movie Chris calls to Father Dyer from her car and says, "I thought you'd like to keep this". If you read her lips she just says, "I thought you'd like this". (02:05:25)
Continuity mistake: Father Merrin prays with his Rosary Beads as Chris brings him a cup of coffee. At first the cross is up near his hand but in the next shot it's dangling close to the floor. (01:41:55)
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)
Revealing mistake: As Father Merrin puts his hand on Regan’s head, nasty green vomit starts to come out of her mouth. If you look closely when she turns her head towards him you can see the hose under her ear that proceeds under a false cheek and into her mouth.
Continuity mistake: Father Merrin says the Lord's Prayer as he stands at the edge of Regan's bed. At first she's sticking her tongue out at him then in the next shot she's not. (01:45:20)
Continuity mistake: Det. Kinderman throws his napkin down flat on the table in front of Chris as he gets up. In the next shot it has been rearranged on the table. (01:15:40)
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)
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.