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)????
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.