Mark Zuckerberg's college roommate, Joe Green, turned down an offer to help Zuckerberg start Facebook.
When the two were at Harvard, they created a Hot-or-Not style website called Facemash, which got the pair in trouble with the university. When Zuck asked Green to help him with Facebook, Green's dad discouraged his son from doing another project with Zuck.
Had Green joined the company in those early days, he would have gotten about about a 5% stake, he thinks, which today would be worth about $7 billion.