"In the fourth quarter of the Cincinnati-Green Bay game, Cedric Benson was tackled on third down, in Green Bay territory and one yard short of the first down marker, with about 5:10 left on the clock. Cincinnati took their time deciding what to do and finally sent out the punt team. The play clock was down to about five seconds, and Green Bay's defense was sprinting off the field as the punt return team took their place. At this point officials reset the play clock to 25 seconds, and the clock ran all the way down to about 4:10 before Cincinnati took a delay of game."
I did some digging and this appears to be entirely accurate. The Bengals ran a play with 5:20 left. It was a one-yard run that stayed in bounds. The play clock is supposed to start counting down from 40 seconds immediately when the play stops. The Bengals then took a delay of game at 4:09 (which was declined by the Packers).
The entire section on delay of game in the rulebook (4-5-6, pages 21-22) doesn't account for any time where the game clock would keep running while the play clock is stopped. If the play clock stops for some reason -- the game clock should stop -- then the play clock should be reset to the same number of seconds (article 3, option c in the rule). If there are less than 10 seconds and the play clock is stopped, it should be reset to 10 seconds. Not 25. There are seven examples given (in article 2) as to when the play clock should be set to 25, and they are all times when the game clock is completely stopped.
When sifting through these rules and the entire section on timing -- in addition to racking my brain for any reason to send the play clock back up to 25 -- it appears the Packers were legitimately shorted somewhere in the ballpark of 20 seconds.
That's from Matt Snyder at Fanhouse.