AP

Steelers defensive lineman Brett Keisel thought he had ended the Bengals’ first series on Sunday with a third-down sack. And then a flag came down that left the Steelers stunned.

After Keisel sacked Bengals quarterback Andy Dalton on third down, he got up and did his signature sack celebration, in which he pretends to be shooting an arrow. One problem: Keisel put his knee on the ground while doing the celebration, and any player who goes to the ground as part of a celebration is supposed to get a 15-yard penalty. Sure enough, the officials threw the flag and the Bengals got 15 yards and a first down.

As shown by NFL Films on Inside the NFL this week, none of the Steelers saw that penalty coming. Keisel looked shocked, Steelers coach Mike Tomlin was captured on film yelling at an official in disbelief, and Steelers running back Jonathan Dwyer said, “He does that every time he gets a sack.”

If Keisel does it every time, the NFL rules say he should be penalized every time. The NFL has had that rule for years, which is why Vikings defensive end Jared Allen stopped putting his knee on the ground during his signature “calf roping” celebration a couple of years ago.

But while the Steelers should have known the rule, the officials also could have handled it better: For some reason, the officials didn’t announce the penalty until both the Steelers’ defense and the Bengals’ offense had gone to the sidelines and the punt teams had taken the field. That led to a delay before the Bengals’ offense and Steelers’ defense re-took the field, and then there was another delay while the officials conferred about something before they finally put the ball in play again for the Bengals’ next snap. It’s unclear what caused the officials’ delay.

The only thing that is clear is that any Steelers who didn’t know before know now: Don’t go to the ground to celebrate.