How are the regulators going to explain this one? MF Global, the failed firm whose Chairman and CEO is Jon Corzine, has already destroyed the wealth of its investors and roiled the banking world. But now we are learning that it may have lost customer funds as well.

A major Wall Street broker in derivatives markets with $41 billion in assets, MF Global filed for bankruptcy on Monday after Mr. Corzine made disastrous bets on bonds issued by European governments. It initially appeared...