Originally Posted by Sunny Day

Banks take in deposits in dollars, say $100 they only have to pay out $100 USD when the customer wants his money back. Banks are required to keep 10% of the deposits. The rest they lend out in loans. If they need more than 10% on any given day, the Federal Reserve gives them a short-term loan. So banks never run out of money. It has happened a few times, but with FDIC insurance the depositors get all their money back.

MTGOX claims to trade 80% of all bit coin trades. The problem is when they sell Bitcoins they need to keep 100% of the sales as reserves. They do charge fees, but the fees alone probably don't cover their overhead and salaries. So even it Bitcoins remained at a stable price they probably couldn't refund customers at 100%.

Since Bitcoins fluctuate in price, that's what going to jam MTGOX up. They started out selling Bitcoins at just a few pennies. Lately Bitcoins have been over $100. It's difficult to sell a product for a few dollars and buy it back for hundreds. That's impossible. Only way they can stay afloat is to constantly bring in new customers paying ever increasing prices.
Normally that business model is called a Ponzi scheme.
The only other thing that could save the day is Bitcoin buyers actually buy goods and services from other customers but it looks like that's a small percentage of Bitcoins.