What most people don't understand is that when you open a brokerage account you allow your assets to be used to "borrow, pledge, repledge, transfer, hypothecate, rehypothecate, loan, or invest any of the Collateral"

Money quote:

...virtually unlimited leverage via the shadow banking system, in which there are practically no hard assets backing the infinite layers of debt created above, and which when finally unwound, will create a cataclysmic collapse of all financial institutions, where every bank is daisy-chained to each other courtesy of multiple layers of "hypothecation, and re-hypothecation."

MF Global: The SERIOUS Issues Reach Mainstream Media