(Reuters) - An error by S&P Dow Jones Indices may have contributed to volatility in shares of Morgan Stanley ( MS ), Pfizer ( PFE ), PNC Financial Services Group ( PNC ) and other stocks in recent days, traders said.

At issue was a list of pending changes to the Dow Jones U.S. Dividend 100 Index that S&P Dow Jones Indices released late on March 1, naming 10 companies slated for addition to the index and two for deletion. The changes were to take effect on March 18.

A total of 23 stocks were affected by the changes, according to trading desk notes obtained by Reuters.

A S&P spokeperson said an unspecified error led to the wrong names being provided to clients.

S&P Dow Jones issued a replacement pro forma list with a total of 11 additions and three deletions after the market's close on Tuesday.

Morgan Stanley ( MS ) and PNC were among the companies that were to be added to the index on the original list.

Their shares rose on Monday, but gave back those gains on Wednesday. By contrast, shares of Pfizer ( PFE ), which were slated for deletion on the initial list, fell on Monday but rose on Wednesday when the revisions were published and it was clear that it would remain in the index.

Trading volumes for all three companies were 50% to 80% higher than their 90-day averages this week.

Morgan Stanley ( MS ) and PNC declined to comment. Pfizer ( PFE ) did not immediately respond to a request for comment.

"This is not that common an occurrence," said Bryan Armour, an analyst at Morningstar who tracks exchange-traded funds.

The error triggered a scramble on trading desks to help their own clients unwind trades made in anticipation of what proved to be incorrect upcoming changes, traders said.

The Dow Jones U.S. Dividend 100 Index is designed to offer investors a benchmark composed of stocks in companies with strong fundamentals and a record of paying high dividends consistently. It is used by some index funds to build portfolios.

Some traders also seek to buy on announcements of upcoming changes before they take effect and any asset management firms using it as a benchmark must adjust their own portfolios.