Graham Bowley and Edward Wyatt for the New York Times:

After a weekend of analysis, many specialists at the major exchanges no longer believe that a single large sell trade in one stock, like that of Procter & Gamble, was the trigger, according to the people familiar with the investigation. Instead, they suspect that a mismatch in rules between the older New York Stock Exchange and younger electronic exchanges set off a frightening sequence of events.

It is not known exactly what caused the initial sell-off in the blue chips, but investigators say the earliest sign of trouble they have found was a sudden drop in the value of a futures contract on the Chicago Mercantile Exchange, based on the Standard & Poor’s 500-stock index. That pushed down a broad array of stocks in that index, all of them traded on the New York Exchange and other major exchanges, and sent many stocks on the New York Exchange into slow mode.

Now it wasn’t a typo, but rather a much larger problem. All the exchanges are not in sync, which can cause quite a ripple in stock prices it would seem.