I spent a year neglecting my PhD and churning out code for this "startup" while the founders moved gradually further away from the idea of a social network and towards something akin to the million dollar home page. When I eventually quit, they couldn't find anybody naive enough to replace me and none of the other three guys (2 marketing, 1 graphic design) knew the first thing about programming, so the whole thing imploded.

I learned some good lessons from the experience, though.

The reddest flag - "We expect 95% of college students will be active users within 4-8 months." And no insight into how they feel they can do this. Claiming market share of any kind is always tough. In 4-8 months, even more dubious.