Continuity mistake: Father Merrin says, "Hear my prayer" as he puts his hand on Regan's head. If you look closely the vomit first comes out when her head is facing the right and it totally soils her blue nightgown. In the next shot with her head facing left there's nothing on her blue nightgown. (01:48:05)????
Continuity mistake: As the Language Lab Director plays the tape of Regan backwards for Damien he turns on the wrong tape recorder. There's no indication that this is not the first time he's heard the tape and he uses the recorder on the left to listen, and is not dubbing anything onto the recorder on the right. When we hear the backwards tape he uses the recorder on the right which does not have the right tape spooled up and doesn't have an overdub of the original to play backwards either. (01:35:40)????
Continuity mistake: Lots of stuff moves as Damien waits on the couch in Chris's living room as she pours him a drink. The flower pot on the back wall under the ceramic pumpkin moves to the left and the couch he's sitting on moves a couple of feet to the left. Later everything moves back. (01:34:00)????