In one of the most gut-wrenching hours in Wall Street history, the Dow Jones Industrial Average (DJIA) plunged almost 1,000 points in early afternoon trading on May 6, 2010 - dropping 600 points in only five minutes - before recovering to close down 348. This move represented the biggest one-day point decline on an intraday basis in Dow Jones history. The DJIA The drop became known in the industry as the "Flash Crash" of May 6.
Fears about the spread of the European debt crisis dragged on stocks through the early afternoon. It was believed that high-frequency trading exacerbated the move lower. There was also talk that a trade by Universa, a hedge fund advised by Nassim Taleb, author of "Black Swan: The Impact of the Highly Improbable," led traders on the other side of the transaction — including Barclays Capital, the brokerage arm of British bank Barclays PLC — to do their own selling to offset some of the risk.
Some stocks plunged dramatically, including Dow component Procter & Gamble (PG, Fortune 500), which dropped 37 percent to $39.37 per share from the close of $62.12. The consumer products maker recovered most of that loss by the close, ending just 2 percent lower.
In the days that followed the May 6, 2010 market decline, both the Commodity Futures Trading Commission (CFTC) and Securities and Exchange Commission (SEC) investigated what had occurred during the trading day.SEC chairman Mary Schapiro said in prepared testimony for a hearing in Washington that the rapid drop “undermined confidence in the integrity of the financial markets.” She added that changes needed to be considered to prevent severe disruptions in the future.
In the end, Nasdaq OMX Group canceled more than 10,000 trades after the event. Euronext’s New York Stock Exchange also agreed to cancel “clearly erroneous” trades after hundreds of stocks and exchange-traded funds (ETF).
In May of 2010, major stock exchanges agreed to introduce temporary trading limits on individual stock moves. The leaders of six major exchanges agreed to a structural framework, to be refined over the next day, for strengthening circuit breakers.The New York Stock Exchange, NASDAQ, BATS, Direct Edge, ISE and CBOE — and the Financial Industry Regulatory Authority (FINRA) to discuss the causes of the so called "flash-crash" in the stock market May 6, 2010, the potential contributing factors, and possible market reforms.
Most of the 50 U.S. exchanges regulate themselves and design their own tools for slowing or halting trading.
In July 2010, NYSE Euronext told the SEC it recommends that regulators seek out a broad-based stock index like the Standard & Poor's 500 to underlie a new circuit breaker. BATS Global Markets also expressed support for tying a market-wide circuit breaker to the S&P 500 index. NYSE Euronext has said it supports tightening the existing levels at which such a marketwide trading halt would occur. 
On April 21st, 2015 UK trader Navinder Singh Sarao was charged in connection to the "Flash Crash" of May 6, 2010. He was charged in the U.S. District Court for the Northern District of Illinois with wire fraud, 10 counts of commodities manipulation, 10 counts of commodities fraud, and 1 count of spoofing.
CFTC - SEC Report
A report about the market decline on that date, jointly written by the staff of the Commodity Futures Trading Commission (CFTC) and Securities and Exchange Commission (SEC) and released in October of 2010, pointed to a large trade made by mutual fund company Waddell & Reed Financial Inc as a trigger for the market plunge that day. The report didn't cite Waddell by name, but said an algorithm the company used to execute a large sell order of the E-mini Standard & Poor's 500 futures contract was based solely on the volume in the futures market "without regard to price or time." The report said volume that day wasn't a good indicator of market liquidity. The sell order of 75,000 contracts prompted a selloff and set off a chain reaction, the report said.
Waddell said it didn't intend to "disrupt" the market through its trading.
Data firm Nanex, LLC questioned regulators' finding, suggesting Waddell's algorithm actually did factor in price because data showed a slowdown in selling by Waddell during the market's steepest decline.
Navinder Sarao was arrested on April 21, 2015 by Scotland Yard in London on charges of market manipulation and spoofing. The CFTC alleged that on May 6, 2010, the day of the so-called Flash Crash, Sarao was active in the E-Mini S&P futures market on the CME Group. The agency alleged that Sarao's use of a "dynamic layering technique" contributed to an order book imbalance between buy-side and sell-side orders. It also claimed that he used the layering technique continuously from 11:17 am to 1:40 p.m. on May 6, 2010, as well as the spoofing technique between 12:33 p.m. and 1:45 p.m., creating downward pressure on prices in the market, especially given the sizes of orders he was placing. The CFTC said that Sarao made $879,018 in net profits in the E-Mini that day and has made more than $40 million between 2010 and 2014.
The agency also alleged that he used the strategies on several days in 2010 and into April 2014.
On April 22, 2015 CME Group released the following statement in response to the Navinder Singh Sarao case: "Nothing is more important to CME Group than the integrity of our marketplace. Following the Flash Crash on May 6, 2010, together with other regulators, we did a thorough analysis of all activity in our markets during the Flash Crash, and concluded – along with regulators – that the Flash Crash was not caused by the futures market. If new information has come to light, we look forward to reviewing it with the Commission. We fully support the CFTC's actions to prosecute those who attempt to engage in fraud or manipulation. We are prohibited by law from releasing information about any individual's trading behavior, including Mr. Sarao's, so we are unable to comment further at this time." 
- Securities And Exchange Commission, Release No. 33-9123; File No. 265-26, Commodity Futures Trading Commission
- CFTC, Criminal Complaint Against Navinder Singh Sarao, Commodity Futures Trading Commission
- U.S. District Court for the Northern District of Illinois
- Glitches send Dow on wild ride. CNN.Money.
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- Comparing May 6 2010 To The 1987 Stock Market Crash Intraday. Daily Markets.
- SEC Said to Consider New Rules as It Investigates Market Plunge. Bloomberg Businessweek.
- SEC Says Market Plunge ‘Unacceptable’. The Globe And Mail.
- Nasdaq says more than 10,000 trades have been canceled. The Globe And Mail.
- Exchanges to Retool Circuit Breakers. The Wall Street Journal.
- Press Release. SEC.
- NYSE Backs S&P 500 for Marketwide Circuit Breaker. The Wall Street Journal.
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- Regulators Defend Report on 'Flash Crash'. WSJ.com.
- Illinois data firm points finger toward high-frequency traders in May 6 flash crash. The Kansas City Star.
- CME Group Statement. CME Group.