Those buying shares during these periods of unusual trading generated gains of as much as 40 percent.

The unusual trading occurred on days when no news or other public announcements occurred that might have affected the stocks.

Some unusual trading did in fact occur, but each such trade proved to have an innocuous explanation.

"In the interval, after the analysts know the news, but before the public knows it, there is a great deal of unusual trading," he said.

Options exchanges are wary of unusual trading ahead of deals.

For those who bought shares during these periods of unusual trading, quick gains of as much as 40 percent were possible.

Measuredmarkets provides examples of unusual trading to institutions, individuals and a regulatory organization in Canada.

Officials at the companies said that they were unaware of unusual trading in advance of the deals and declined to speculate on reasons for the action.

This time the unusual trading showed up both in the stock of Golden West Financial and in its call options.

As of last night, no one had come forward to acknowledge having bought the stock by mistake, and Nasdaq market officials were investigating the unusual trading.