In the second season of Supernatural in the Hollywood Babylon episode Sam and Dean are on set for a movie called Hell Hazers. Towards the middle of the episode they show a preview for the movie. In the preview you see a clip of one of the characters from the movie holding her camera phone up so as to see the spirits. Another character is then shooting them with a shotgun.

This is a slip up because the 'writers' of the movie dont make that part of the script until the end of the episode. Dean and Sam do this when fighting off the evil spirits. Then the writer puts a scene in his movie. Sam comments "You find out there is an after life and this is what you do with it?"

How could they already have a clip of that in their preivew if it hadnt happened yet?