(FORTUNE Magazine) – The unofficial rule followed by most money market funds is simple: investors should be able to take at least one dollar out for every dollar they put in. Until recently, fund sponsors, including companies such as Value Line and BankAmerica, have bailed out funds threatening to "break the buck" -- i.e., fall below one dollar per share in net asset value. In each case the sponsors either bought out the derivatives that spiked share values or infused additional cash, even though they were under no legal obligation to do so. In each case, that is, except one -- Denver's Community Assets Management. Community Assets president John Backlund set a worrisome precedent by allowing his firm's small fund to go bust. The fund was heavily invested in derivatives, which likely became illiquid as interest rates rose. Expect the SEC, which is looking at funds that use derivatives, to strengthen disclosure guidelines. Still, the majority of investors are not at risk. "Our inspections to date suggest that the use of derivatives by most funds is limited," SEC chairman Arthur Levitt told Congress. And most sponsors are in better financial shape than Community Assets, which says it could not raise the necessary capital to save the fund. According to industry newsletter Fund Action, sponsors have spent at least $674 million bailing out mutual funds; but allowing them to sink obviously destroys reputations and alienates potential investors.