Ah here we go:
After almost five months of investigations led by Gregg E. Berman,[7][8] the U.S. Securities and Exchange Commission (SEC) and the Commodity Futures Trading Commission (CFTC) issued a joint report dated September 30, 2010 and titled “Findings Regarding the Market Events of May 6, 2010” identifying the sequence of events leading to the Flash Crash.[9]
The joint report “portrayed a market so fragmented and fragile that a single large trade could send stocks into a sudden spiral,”[10] and detailed how a large mutual fund firm selling an unusually large number of E-Mini S&P 500 contracts first exhausted available buyers, and then how high-frequency traders (HFT) started aggressively selling, accelerating the effect of the mutual fund’s selling and contributing to the sharp price declines that day.
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HFTs began to quickly buy and then resell contracts to each other – generating a “hot-potato” volume effect as the same positions were rapidly passed back and forth. Between 2:45:13 and 2:45:27, HFTs traded over 27,000 contracts, which accounted for about 49 percent of the total trading volume, while buying only about 200 additional contracts net.
http://en.wikipedia.org/wiki/2010_Flash_Crash
I wonder if this was done on purpose as it only benefited the democrats while an election was underway and McCain fell for it.