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: 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: In his office as Chris talks to Doctor Klein the boom mike's reflection can be seen moving around in the metal light behind him. (00:34:45)
Continuity mistake: Father Karras eats dinner in his mother's apartment and the curtain on the far side of the table moves without being touched the second time we see it. (00:20:25)
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)
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)
Revealing mistake: When Father Karras sprinkles holy water on Regan, they forgot to paint her ear lobes and it looks terrible. (01:32:30)
Revealing mistake: As Father Merrin speaks to another archaeologist in the beginning of the movie the clock on the wall stops between ticks and he turns around to look at it. It's easy to see that the clock pendulum has been pushed up against the window from behind in order to make it stop. (00:06:45)
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: 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)
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.
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)
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)
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.