This entry on TV Tropes is known as Missing Episode. Given the site’s roots as discussing the narrative devices of TV, it’s obvious why this trope is called what it is. However, as TV Tropes evolved to cover all media, this trope naturally was expanded to give all examples of media.
Missing Episode refers to the phenomena of an episode of a TV, or for that matter any work that is no longer publicly available. There are a variety of reason why an episode might be missing. One example could be that the content is considered offensive, such as due to racial insensitivity, violence, sexual content, and so on. Another could be an episode that seems insensitive following a tragedy such as a natural disaster, mass shooting, or the like. Others have to do with legal issues; the most common legal issue is TV shows that license music for the original run, and being unable to license it for reruns or home media distribution, and not releasing it in any form because changing or removing the music would ruin the episode. Another reason is that the episode does not exist anymore.
To expand on that last point, many films from the beginning of cinema are lost forever. Martin Scorcese’s Film Foundation, an organization devoted to film preservation, estimates that over 90% of films from the silent and early sound eras are lost. Even some of the the most popular actors of the time have most or all of their filmographies lost. The reasons are diverse. First of all, many studios assumed that the film lost value after their theatrical runs ended. In a world before theatrical reissues, TV broadcasts, and home media, few to no people assumed people would decades later, let alone the follow year, would want to see them; therefore, studios would simply discard or destroy the films, especially if they lacked the money to store them. Not only that, films were shot on film stock made of nitrate, which was flammable and fragile and would disintigrate or catch fire if they were not properly stored; it didn’t help that studios would intentionally destroy the films to take financial advantage of the silver therein. Another reason is that many early sound films used a sound-on-disc system. If the discs became lost or damages, the films would be considered worthless and discarded.
However, some films even if they did survive, are lost in their original forms. Often scenes would be cut out before original release or before reissues, and discarded and/or destroyed.
My favorite movie Fantasia is lost in its original form. It was originally released as a roadshow engagement at the length of 124 minutes. After the financial failure of that released, it was rereleased in standard theaters recut to 81 minutes, removing the Tocatta and Fugue in D Minor segment, and all of the introductions by Deems Taylor. Later releases restored the Tocatta, but Taylor’s introductions were kept to a minimum. In 2000, for the film’s 60th anniversary, All of Taylor’s interstitials were restored, but audio tracks were damaged beyond use; voice actor Corey Burton was hired to redub Taylor’s dialogue.
A Star is Born (1954) is partially lost. It had a test screening at a runtime of 196 minutes. It was cut down to 181 minutes for the premiere. However, theaters complained about the length, saying it would limit showing, and therefore, profits. As a result, Warner Bros. cut the film to 154 minutes. Director George Cukor and star Judy Garland were outraged. Cukor refused to see the film again, saying that it hurt to watch the recut version. Garland claimed that the fact that several crucial scenes were cut, lead to her losing the Oscar for Best Actress. Ironically, this attempt to increase profitability lead to the film losing money. In 1981, a restoration proved partially successful, extending the length to 176 minutes, and replacing lost scenes that couldn’t be found with photos of the scenes being shot, to give an idea of what they would have looked like.
Many American soap opera episodes are lost, as the owners of them did not preserve them. It was not until the middle of the 1970s, that they began preserving the episodes. The exceptions are Dark Shadows, which has only one lost episode, and Days of Our Lives and The Young and the Restless, which have preserved all of their episodes before it was common to do so.
Many talks show and game show episodes are lost because the tapes were reused, due to videotape being very expensive. This practice ended in the late 1970s.
The BBC is famous for discarding many of their programs. This practice is known as wiping. It was not until 1978, that they developed a policy of preserving all their material. The reasons were varied, but included issues such as cost, making room for new programs, and deals with talent unions to limit or forbid reruns from talent unions on the rationale that reruns might put them out of business. The most famous examples of this policy is Doctor Who. Many early episodes are lost forever, but occasionally, lost episodes did turn up.
With regard to anime, many episodes are missing often because of content reasons, that are offensive to Western sensibilities or deemed inappropriate for children. One anime series is, however, lost in its original form because of of changes made when it was imported for dubbing in America. That would be Astro Boy. Adter NBC recut the episodes, they offered to send the original film elements back the rights holders in Japan; the studio, however, refused to accept them because of financial issues, and they told NBC to do whatever it is that they do with film elements that they can’t or don’t want to keep. NBC discarded them, and now all versions of the show are made from NBC’s version.
I even have a lost film of my own. In 1999, me, my brother, my sister, and my father created a short film called The Giant Pikachu. I lost the VHS tape is was saved on.