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: 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: 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)