Barry Ritholtz has the answer to the riddle.

I refer of course to the debacle that is MF Global. How on earth could $633 million in customer accounts simply disappear?

As it turns out, quite legally.

The regulations governing these customer accounts are 25 plus years old, according to a few insiders I spoke with. They gave the firms an ability to hypothecate (lend) client money, so long as it was only used to legally purchase investment grade sovereign debt.

So that was what MF Global did.

As originally conceived, client monies were only supposed to purchase US Treasuries. However, so as to not offend trading partners (and other reasons), the regs were written so as to include any “investment grade sovereign” in the rules. Hence, AA rated European sovereign debt, despite the obvious fact that in 2011 they are obviously not the equivalent of US Treasuries, technically qualify. Whether this violates the obvious spirit and intent of the law will be for a judge to decide.