Reportedly, well-respected stock trading company Knight Capital Group, accidentally released test software used to "verify that their new market making software functioned properly, into NYSE's live system".
From the article in The New York Times, explaining the events:
The problem on Wednesday led the firm's computers to rapidly buy and sell millions of shares in over a hundred stocks for about 45 minutes after the markets opened. Those trades pushed the value of many stocks up, and the company's losses appear to have occurred when it had to sell the overvalued shares back into the market at a lower price.
The company said the problems happened because of new trading software that had been installed. The event was the latest to draw attention to the potentially destabilizing effect of the computerized trading that has increasingly dominated the nation's stock markets.
Until this week, Knight had been one of the biggest beneficiaries of the evolution of the market, helping clients trade in and out of stocks at high speeds. The firm was responsible for 11 percent of all trading in American stocks between January and May, according to Adam Sussman at the data company Tabb Group.