In the span of a few minutes this afternoon, the Dow lost nearly 1000 points, then gained most of it back. There were lots of reports saying that fears about Greece triggered the sell-off.
But mistakes by traders or glitches in computerized trading systems may have played a role as well.
The WSJ reports that a trader may have accidentally placed an order to sell $16 billion in futures tied to stock indexes, when he meant to place the order for $16 million.
And there were "a number of erroneous trades" during the minutes when the market was plunging, a spokesman for the company that owns the New York Stock Exchange told Bloomberg.
The FT's Alphaville blog points out that Procter & Gamble shares fell by more than 20 percent — about three times as much as the Dow — before regaining almost all of the ground it lost, and says the decline may have been related to a "technical screw up." And Barron's notes that shares in Accenture — which opened and closed the day above $41 per share — traded for one cent per share at one point today.
Here's a chart that shows Procter & Gamble (in blue) compared to the Dow (in red) this afternoon: