As head of the Commodities Futures Trading Commission, Brooksley Born was concerned about secretive trading practices of derivatives.  She wanted more oversight of the derivatives market.  But Alan Greenspan, Larry Summers, and others wouldn't listen--and blocked her attempts to put more regulations into the financial system.  When 2008 hit, her worst fears started coming true, as she tells Frontline:

It was like my worst nightmare coming true. I had had enormous concerns about the over-the-counter derivatives [OTC] market, including credit default swaps, for a number of years. The market was totally opaque; we now call it the dark market. So nobody really knew what was going on in the market.

And then it became obvious as Lehman Brothers failed, as AIG [American International Group] suddenly appeared to be on the brink of tremendous defaults and turned out had been a major credit default swap dealer and needed hundreds of billions of dollars to keep it alive, the contagion in the marketplace from those failures brought many, many of our biggest financial services companies to the brink of collapse. And it was very frightening.

Born's story is at the center of a new Frontline documentary called The Warning.  Here's a preview:

You can watch the full program here.