May 6 2010. Dow falls more than 1,000 points intraday, including a drop of P&G from around $60 to (according to some accounts) below $40. The Dow recovered most of the falls quickly, but these trades are now part of the historical time series.
Banks and others using risk management tools often back-test their models against historical data to see how whether the models capture past market movements in estimating potential future market movements. This blip may appear as an anomaly in these tests for some time.
(It’s more typical for the tests to use only closing prices rather than intra-day prices. However, this actually reflects a weakness in the typical models and is only a fortunate escape from today’s problems)