Supernatural Musical Episode: A Discrepancy
Fast forward to the special 200th episode:
The Winchesters discover that not only is their life a book series and a TV show; it is now a musical put on by an all-girls private school (10x05: Fan Fiction). The production includes key points from the story, i.e. losing Mary (the first time), Bobby in a wheelchair, classic "BM" scenes (which specifically stands for "Boy Melodrama", not "Bowel Movement", just FYI).