Any
television pilot episode that doesn't make it to air. Such
episodes are usually
canned and left to rot, unless rescued by a determined
fan. Some pilots don't make it into
series form at all, and are rather like
nodeshells - some attempt was made, but is now lost in the
void.
Some famous unaired pilots:
"Gilligan's Island" (1964) - An unaired "lost" pilot, until TBS brought it to the small screen in 1992. Featured three actors that didn't make the final cut: John Gabriel as The Professor, Nancy McCarthy as Bunny, and Kit Smythe as Ginger.
"M3K" (2000) - an excellent treatment of a modern-day retelling of Alexandre Dumas' story, The Three Musketeers. In a world where guns are illegal, and everyone carries swords, the King of America's sworn protectors, the Musketeers, are all that stand between the corrupt Cardinal Richelieu and the throne. And only the renegade Musketeers, Athos, Porthos, Aramis, and D'artagnon have the courage to defend their King and country from their enemies. An interesting twist in this plot: D'artagnon is a woman, and the sexual tension makes for some interesting sword play of another kind... Unfortunately, despite the quality of the writing and production (reminiscent of the Highlander series), the show got caught in production politics, and was killed before it even aired.
"Buffy The Vampire Slayer" (1996) - a half-hour pilot filmed by Joss Whedon, creator of the movie by the same name. Featured many of the same actors and actresses, except for Willow. Joss used this pilot to shop his series around until it got picked up by WB.