Traders in the S&P 500 pit at the Chicago Board Options Exchange on May 6, 2010. (Terrence Antonio James/Chicago Tribune)

By Greg Burns | One of the wildest 20 minutes in Wall Street history spilled into Chicago’s major markets Thursday, prompting one exchange to declare a series of trades “clearly erroneous,” and another to suggest that a hot rumor was wrong.

The CBOE Stock Exchange invoked a government-sanctioned rule to unwind 18 trades made in the stock of Chicago’s Accenture Plc that all took place within a few minutes Thursday afternoon at the price of a penny per share.

Accenture closed down $1.08 at $41.09, and no news from the company would have justified those rock-bottom trades.

Rather, so many orders were flowing into the all-electronic exchange that its market-making “book” was “exhausted down to the final quote at a penny,” noted David Harris, chief executive of the stock mart.

Under regulations common to all U.S. securities exchanges, trades occurring at those “erroneous” prices either get cancelled or adjusted to a reasonable price, Harris said. Regulators consider sorting those outlying trades afterward a better approach than trying to stop them before they happen.

Meantime, CME Group sought to squelch a rumor that a bank trading system ran amok in its e-mini stock-index futures contracts.

In an unusual statement issued Thursday evening, the Chicago-based company said stock-index trading by Citigroup Global Markets Inc. “does not appear to be irregular or unusual in light of market activity today.”

A CME spokesman declined to elaborate, except to say, “Our markets worked fine.”