Revealing mistake: You can see wires in the scene where Regan levitates above her bed in the exorcism scene, lifting her up. The way they pull on her dress is also visible.
Revealing mistake: In the scene where Sgt Howie hits McGregor (who is dressed as Punch) with a candlestick, watch closely and you can see that the blow from Howie is aimed into the padded section on the back of the punch costume, rather than at the back of McGregor's head.
Revealing mistake: Right at the end when John is dying look at the texture of his blood. It is way too thin, almost like coloured water.
Revealing mistake: When Jack Hawkins punches through the door it is very apparent that it's made of very thin wood to make it much easier to hit through.
Revealing mistake: When Mrs. Wadsworth is whipping the babysitter, the babysitter's stunt double is obvious due to the style and color of her hair.