Continuity mistake: Sitting in front of the fireplace at her party, Father Dyer tells Chris that Father Karras' mother had died the previous night. The candle in the candleholder to her right rotates throughout the scene. (00:40:35)
Continuity mistake: At the party, a drunk Burke Dennings calls Karl a, " Bloody, damned, butchering, Nazi pig!" and the chain around his eyeglasses moves on his chest twice between shots. (00:40:45)
Continuity mistake: At the party Father Dyer sings just before Regan walks in and says to the Astronaut, "You're gonna die up there". The music book on the piano is open to the same page but the manuscript paper underneath has been moved between shots. (00:42:35)
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: 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: 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)
Visible crew/equipment: After Chris puts Regan to sleep she walks through an equipment shadow as she goes down the steps and asks the maid, "Is it coming out Willi?" (00:44:10)
Continuity mistake: After Chris puts her to sleep and walks down the steps, Willi the maid is cleaning up Regan's mess on the living room carpet from the previous party scene. We saw a nice close-up of the wet carpet as it happened and now Willi is not even close to cleaning the right spot. In fact she's kneeling right on it, yikes. (00:44:20)
Continuity mistake: After Chris puts her to sleep and walks down the steps, Regan screams and there is a close-up of the lamp flickering on the wall. In the next shot the lighting has changed as a dark shadow appears behind the lamp. (00:44:25)
Continuity mistake: Regan screams, "Mother, make it stop" and the books near the lamp on her desk move to the right between shots. (00:44:40)
Continuity mistake: Father Dyer walks down the hall on his way to Father Karras' room and the florescent ceiling lamp closest to the window is turned off between shots. (00:44:55)
Continuity mistake: Father Dyer walks into Father Karras' room and the Chivas Regal bottle on the floor changes sides between shots. (00:45:10)
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: As Father Karras lays down, Father Dyer shuts all the lights for him. The lamp on his desk is right over the typewriter the first time we see it than later as he turns it off it has been moved to the left. (00:46:10)
Audio problem: In Doctor Klein's office Regan spits at him as he tries to give her an injection. We hear it but if you look closely nothing comes out of her mouth. (00:47:10)
Continuity mistake: Dr. Klein talks to Chris about what Regan's test results indicate and she's touching her face with her left hand. In the next shot from the front, she's touching her face with her right hand. (00:48:05)
Continuity mistake: Dr. Klein gets socked in the mouth by Regan and blood drips out of his nose. The next time we see him, and in all the subsequent shots in this scene, there's no blood on his face. (00:53:40)
Continuity mistake: After he gets socked in the mouth, Dr. Klein gets ready to give Regan another injection and the lamp near the window and the curtain behind it move between shots. (00:54:00)
Continuity mistake: Dr. Klein and Dr. Taney arrive at the house and go up to Regan's room to see her for themselves. We see inside the room and the two doctors, Chris and Sharon are the only ones there but as they try to give her an injection, Karl appears from out of nowhere restraining her. (00:54:05)
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.