A notorious part of the Jerry Lewis story is "The Day the Clown Cried," a movie in which he played a clown working in a Nazi death camp. The movie has never been in a theater and by all accounts never will be.
A clip with that link has Lewis ackowledging the film did not turn out the way he wanted and no one will see it. But in his 1982 memoirs, Lewis blames litigation for tying up the film, and still hoped "I can go back to Stockholm and shoot three or four more scenes."
.... the story is timeless. I can release it ten years from now, and it will hold up. One way or another, I'll get it done. The picture must be seen, and if by no one else, at least by every kid in the world who's only heard there was such a thing as the Holocaust.
Time, clearly, has dimmed Lewis's belief in the film. But because of what it is, and because it has not been seen by most people, the Internet has been buzzing over the YouTube clips above. They appear to be a German look behind the scenes during the making of the movie, and so add to the fascination about what the movie itself might have been.