The Dow Jones Industrial Average dropped 998.5 points during the 2010 flash crash, its largest intraday point fall at that time.
On May 6, 2010, major US stock indexes plunged within minutes before recovering much of the loss. The Dow's decline amounted to about 9% from its previous close, although it finished the day down 347.8 points. Prices in individual stocks and exchange-traded products briefly became highly dislocated.
Investigations by US regulators concluded that a large automated sell order, combined with stressed market conditions and high-frequency trading activity, helped produce the extreme volatility. The event led exchanges and regulators to strengthen safeguards, including circuit breakers and rules for clearly erroneous trades.
The 998.5-point figure describes the intraday decline, not the final closing loss. That distinction explains why another number is often quoted in summaries of the same event.