Mr. Toad's Wild Ride

A computerized selloff possibly caused by a simple typographical error triggered one of the most turbulent days in Wall Street history Thursday and sent the Dow Jones industrials to a loss of almost 1,000 points, nearly a tenth of their value, in less than half an hour. It was the biggest drop ever during a trading day.

The Dow recovered two-thirds of the loss before the closing bell, but that was still the biggest point loss since February of last year. The lightning-fast plummet temporarily knocked normally stable stocks such as Procter & Gamble to a tiny fraction of their former value and sent chills down investors' spines.

"Today ... caused me to fall out of my chair at one point. It felt like we lost control," said Jack Ablin, chief investment officer at Harris Private Bank in Chicago.

No one was sure what happened, other than automated orders were activated by erroneous trades. One possibility being investigated was that a trader accidentally placed an order to sell $16 billion, instead of $16 million, worth of futures, and that was enough to trigger sell orders across the market.

Whatever started the selloff, automated computer trading intensified the losses. The selling only led to more selling as prices plummeted and traders tried to limit their losses.

"I think the machines just took over. There's not a lot of human interaction," said Charlie Smith, chief investment officer at Fort Pitt Capital Group. "We've known that automated trading can run away from you, and I think that's what we saw happen today."