In March 1951, a pilot episode was filmed for I Love Lucy but it never aired. It wasn’t until 1989 that a copy of the unaired pilot was found.
I’ve started putting together posts listing the status of existing episodes of various TV shows.
When Shout! Factory wanted to release the first season of Father Knows Best on DVD in 2008, it couldn’t locate one episode until a collector came forward with a 16mm print.
A black-and-white kinescope of “The Petrified Forest,” a 1955 episode of NBC’s Producers’ Showcase, was recovered in October 1986.