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.

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

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.”

Chiefs

RB CHARLES GOES ON IR: The Chiefs placed Jamaal Charles on injured reserve Tuesday after the star running back tore the ACL in his right knee while making a cut against the Chicago Bears.

Charles had an MRI on Monday that confirmed the extent of the injury.

The Chiefs filled his roster spot by promoting running back Spencer Ware from the practice squad. Ware is a former sixth-round pick out of LSU who spent the offseason with Kansas City.

Panthers

MLB KUECHLY BACK AT PRACTICE: Panthers middle linebacker Luke Kuechly returned to practice for the first time since suffering a concussion in the Carolina’s season-opening win against the Jacksonville Jaguars.

The four-year pro spent four weeks in the NFL’s concussion protocol and missed three games before being cleared to return. The Panthers had a bye last week and travel to play Seattle on Sunday.

Wide receiver Jerricho Cotchery also practiced after missing the last two games with a high ankle sprain.

Defensive end Jared Allen did not practice after pinching a nerve in his back in the team’s Week 4 win against Tampa Bay.

Rams

HOUSE SPEAKER LETTER BLASTS STADIUM PROPOSAL: In Jefferson City, Missouri, the state’s House speaker is gathering signatures for a letter to Gov. Jay Nixon that slams a plan to use state money for a new St. Louis football stadium.

A letter from Speaker Todd Richardson obtained Tuesday by The Associated Press urges Nixon and leaders of a stadium task force to work with lawmakers.

A Nixon spokesman said he hasn’t seen Richardson’s letter and declined comment.

Richardson is the latest of several lawmakers to speak against using taxpayer money for a stadium without legislative or voter approval.