There is an unaired episode of The Simpsons from the first season called "Dead Bart." It was pulled from the production schedule for reasons that were never officially documented. Matt Groening is said to have become visibly upset when the episode was mentioned in a 2000 interview. He confirmed only that it existed and refused to discuss it further.
The plot, as described by someone claiming to have worked in Klasky-Csupo during Season 1 production: the Simpson family takes an airplane trip. The plane hits turbulence. A window breaks. Bart is sucked out of the plane.
The animation quality deteriorates sharply after this point. The remaining runtime — approximately fourteen minutes — depicts Homer, Marge, Lisa, and Maggie grieving. The family is drawn increasingly off-model as the episode progresses. Colors desaturate. The voices, which were reportedly recorded normally, are said to sound wrong in context — authentic grief rather than performed grief.
The final scene shows the family at Bart's grave. The tombstone is properly rendered. The surrounding graves are reportedly inscribed with real names — celebrities, actors, people who were alive at the time of production — along with their actual dates of birth and, in the far right corner of the frame, dates of death. The dates of death are in the future.
This is the detail that has been consistent across every account: the death dates in that final scene. Not random. Specific. Verified, in retrospect, to be accurate.
The original cel animation was supposedly locked in a vault after a single internal screening. Groening kept a VHS copy. No one else is known to have a copy, though several people claim otherwise.
The episode ends with Bart staring at the camera from within the grave. He doesn't look like Bart by this point. He looks like what you'd get if someone described Bart to an artist who had only heard the description and not seen the show. He says something directly to the viewer that is not included in any account.
What he says differs in every telling.
Which is, of course, impossible. Either he says something or he doesn't. Either the accounts are fabricated or they're real. There is no middle position.
Except: people who claim to have seen the episode describe the final line differently. And their descriptions of the rest of the episode match, precisely, in every other detail.