That happened when Lehman Brothers failed in September 2008. The Reserve Fund, a well established money market fund with too many unpaid IOUs from Lehman, could not keep its value steady.

Money market funds take excess cash from investors and use it to purchase short term IOUs from businesses, banks, and other financial institutions. They mimic bank accounts by allowing investors to write checks and promise that their investment s value will not fall.

Since much of the money market funds assets are IOUs from the world s biggest banks, the withdrawals weakened the already shaky global banking system. The Federal Reserve, seeking to stem the growing panic and stabilize the American and international banking system, promptly guaranteed the value of all money market funds.

Even if the Greek government decides to pay wages and pensions by printing its own IOUs or new drachmas, the European Court of Justice will rule that all domestic debts and bank deposits must be repaid in euros. That, in turn, will force a default against Greek citizens, as well as foreign creditors, because the government will be unable to honor the euro value of insured deposits in Greek banks.