For the second week in a row, the N.F.L. acknowledged an officiating error in a nationally televised game.
This time, the problem was twofold: 18 seconds were incorrectly run off the clock late in the fourth quarter Monday night in the Pittsburgh Steelers’ 24-20 victory at San Diego, and none of the game officials noticed it.
In a statement emailed Tuesday, the league spokesman Michael Signora said that “an error by the clock operator” after a touchback on a kickoff allowed the extra time to tick away, and “it is the responsibility of the side judge to supervise the timing of the game.