Star Trek fans know that the pilot episode entitled The Cage, starring Jeffrey Hunter, was not aired on television until long after the show went into reruns, in 1988. What Gene Roddenbury did do however was elongate the show into a two-part episode, The Menagerie, which is widely reputed to be the best Star Trek episode of all time.
One fascinating difference between The Cage and The Menagerie is that from among the famous cast members only Leonard Nimoy appears in The Cage. Its story was only about Captain Pike and the Tolosians. To make it longer, Roddenbury wrote in the rest of the cast, embedding, as it were, the pilot film into the longer The Menagerie. Wikipedia has the whole story.