I can see why it's not on the list because of the changes to the story, but believe it or not The Exorcist is based on a true story. The role of Regan (the main protagonist) was actually a young man (of similar age) by the name of Ronald Edwin Hunkeler, originally from St Louis, but who lived in a suburb of Washington when the events occurred. The "door" which enabled the possession was.. wait for it... an Ouija board.