7. Escape From New York Special Edition - Kansas Salt Mine
The MGM Special Edition of 1981's Escape From New York could not be shown to audiences until 2003 because the negative print had been lost, with nobody really seeming to know why given the relatively young age of the film. However, a salvageable workprint of the 80s classic was incredulously discovered at a Hutchinson, Kansas salt mine film depository. The added scenes essentially introduced us to Snake and developed his character a little more, and though director John Carpenter asserts that they are completely perfunctory scenes, it's still a relief for fans that the print was kicking around in some industrial building somewhere.
Why it was doing that, however, is another question altogether.