Say, on a Monday morning you wake up, and the stake of $1bn you owned in an investment bank has just been revalued at $12million. Unfortunately, you have just bought this appartment for $50million (after all, you have until Sunday night been a billionaire) with a really good super-jumbo mortgage deal. However, your mortgage bank has read the news as well and you receive a margin call. You have no income, since you're retired. Your equity is minus $38million, you get repossessed and go bankrupt.
You'll be seen collecting cans & tins on Wall Street briefly afterwards.