Mercilus was penalized for unnecessary roughness on the hit on which his helmet struck Flacco in the chin, which resulted in a first down even prior to the penalty. The Ravens didn't score on the drive.
On the play, Mecilus and Flacco's heads collided as the quarterback was going down. It's exactly the kind of play defensive players decry as a case where there wasn't anything they could have done to avoid the way the hit unfolded. After the game, Texans defensive end Antonio Smith called the flag for the hit "bogus."
It's Mercilus's first fine for a hit and he'll appeal it.
He's a pass-rusher, though, so it's not likely this will be his last.