For the second week in a row, the NFL 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 the San Diego Chargers — and none of the game officials noticed the gaffe and corrected it.

In a statement emailed Tuesday, league spokesman Michael Signora said that “an error by the clock operator” after a touchback allowed the extra time to tick away, and “it is the responsibility of the side judge to supervise the timing of the game.”

The statement added: “Had the side judge or any of the other six on-field officials noticed the timing error, they could have corrected it.”

In last week’s Monday night game, an official missed a late-game call in Seattle’s 13-10 victory over Detroit, giving the Seahawks the ball even though one of their defenders knocked a fumbled ball out of the end zone, which should be a penalty. That prompted the NFL’s head of officiating, Dean Blandino, to say that the officials blew it.

The most recent Monday night mistake happened after San Diego kicked a field goal to take a 20-17 lead with 2:56 left. The ensuing kickoff sailed out of the end zone for a touchback, so the clock never should have started. Instead, it did begin moving, and by the time Pittsburgh lined up for the first play of its crucial drive, only 2:38 remained.

“When you see a kick go over the end line, never in your wildest dreams would you think the clock is going to start. Neither team was aware of it. ESPN wasn’t aware of it. Nobody was aware of it. The clock operator is the one who should take the hit,” Mike Pereira, Blandino’s predecessor at the NFL, said in a telephone interview.

“It does happen where a few seconds tick off; it doesn’t happen very often where 18 tick off. It shouldn’t happen,” Pereira said. “Ultimately, the officials on the field are responsible. … Everybody needs to be cognizant.”

With back-to-back prime-time problems, it might seem as officials are more prone to miscues these days.

Pereira, though, noted that those miscues are easier to catch now.

“Social media has made it so much more difficult right now, because everyone is tracking everything. All one person needs to do is see it and put it on social media and then the world finds out about it,” he said. “Years ago, something like this may not have been discovered.”

As it turns out, despite the clock fail, the Steelers managed to make it all the way downfield in time to score the go-ahead points on the final play from scrimmage, a 1-yard TD run by Le’Veon Bell.

“Everyone is probably lucky the Steelers won on that last down,” Pereira said.

And, not surprisingly, Pittsburgh coach Mike Tomlin’s take was, essentially: All’s well that ends well.

“It didn’t define the outcome of the game,” Tomlin said Tuesday, “so I’m moving on with my workweek.”

The truth is, of course, there really is no way to know how the outcome of the game could have been different had those extra 18 seconds not been “lost.” Perhaps San Diego would have been left with enough time for a play or two — or a kickoff return for a TD. Or perhaps Pittsburgh would have called things differently on offense with more room on the clock, maybe running time down to nothing, anyway.

Who knows?

Signora’s statement noted: “The performance of the clock operator and game officials will be reviewed per the standard procedure for reviewing every play of every game.”

Each game-clock operator is hired by the league, not the home team, and lives near the stadium hosting a regular-season game. In the playoffs, the league uses operator who don’t live in the area.

Pereira could recall only one issue with a game-clock operator when he was in charge of NFL officiating: A decade ago, a team asked him to replace its clock person. That team? The Chargers.

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AP Sports Writer Will Graves in Pittsburgh contributed to this report.