Of course, a film about possession must have some creepy elements too.
For example, the set of The Exorcist caught fire and burned down without any explanation, causing the film to be delayed for 6 weeks.
Once the film was up and running, things went missing, and during the notorious scene with Regan and the crucifix, the actress that played her mother, Ellen Burstyn, hurt her back and had to be bedridden for weeks, delaying production even further. When production finally restarted, the crew saw the deaths of a night watchman and a special effects expert during principal shooting.
Finally, when the film was completed, the actor who played alcoholic director Denning Burke, died suddenly from complications from a simple flu.