My dad always said we were related to Maureen O'Sullivan, which would have us related to Mia Farrow, who is still alive, but I have not found any connection, even remotely close. I have no idea how that one even started.
My husband's family claims a relation to the Clown Prince of Baseball, Al Schacht, who has a couple rather famous living authors in his tree. I haven't been able to find the connection, but my FIL and his father have an extremely striking resemblance to Al. My FIL and his sister remember Al coming to visit them in Brooklyn and learning he was their dad's second cousin or something. I just can't find it, although we have no idea what my husband's "real" last name was. It was changed at some point when his great-grandfather came from Russia to England.
And another branch of his family claims a link to Sholom Asch. I haven't found the definitive link and I have written to a couple of his relatives who have genealogy sites, but haven't heard back.
Always an if, but, or whatever.
Kath