Deliberate mistake: Father Merrin says, "It is He who commands you", and we hear and see the ceiling crack above his head. In the next shot we see the ceiling above him and they are no cracks in it, but four shots later we see the exact same spot and now cracks appear even though we hear no more cracking over-dubs after the first one. (01:49:15)????
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)????