In a private notice sent to CME Group clearing members on Friday, the trading exchange said it made a revision on Oct. 30 to the November 2013 natural gas futures contract expiration that occurred on Oct. 29.

Without providing a reason, CME Group in its notice revised the November contract’s expiration from $3.496 to $3.497.

“CME Clearing has contacted all affected firms and made the necessary adjustments,” the exchange said.

Tom Saal of INTL FC Stone in Miami was surprised by the news. “I think this is a pretty big deal. There are an awful lot of contracts tied to that price. What is remarkable is that it even happened, based off of how we know the settlement is calculated. I think an explanation is warranted.”