His Dad advised against working with Zuckerberg on Facebook, losing him a 4 - 6% stake in the company that would have been worth at least $3bn today (but clearly Green would have been diluted along the way).
Joe Green is Zuckerberg’s former Harvard roommate, and he and he asked Green to drop out of the Ivy League university to build the social networking site with him, but Green said no because his father disapproved
Green previously told ABC News: 'We'd gotten into a little bit trouble with the previous project ... and my father, who's a professor, was not too happy with the prospect of me getting kicked out of school. Zuckerberg likes to make fun of my dad for this, but we're still very close'.
See the video below: