The unusual trading event on May 6, 2010, was the flash crash.
During the afternoon, major U.S. stock indexes plunged rapidly and then recovered much of the decline within minutes. The Dow Jones Industrial Average briefly lost nearly 1,000 points, about 9%, before rebounding. Some individual securities traded at extremely low prices.
Investigations concluded that automated trading, market conditions, and a large sell order in E-mini S&P 500 futures interacted in ways that sharply reduced liquidity. The U.S. Securities and Exchange Commission and Commodity Futures Trading Commission later described a feedback loop involving high-frequency traders and rapidly changing orders.
The flash crash was not the same as the 2008 financial crisis or the dot-com crash. Those were extended periods of financial or valuation stress. The defining feature here was the extraordinary speed of the decline and partial recovery during a single trading session.