According to Ray Fittipaldo of the Pittsburgh Post-Gazette, Pittsburgh Steelers cornerback William Gay was fined $15,750 for unnecessary roughness on Cleveland Browns quarterback Jason Campbell when "he unnecessarily delivered a forcible blow to his opponent's head and neck."
NFL fines Willie Gay $15,750 for unnecessary roughness on Campbell. "He unnecessarily delivered a forcible blow to his opponent head & neck"
— Ray Fittipaldo (@rayfitt1) November 29, 2013
This play of course happened on the third quarter sack of Campbell that resulted in the ball being fumbled and returned by safety Will Allen down to the Browns one yard-line. The Steelers scored a touchdown on the very next play.
As you can clearly see in the animated gif below, Gay's left hand strikes Campbell right in the facemask. There was no penalty called on the play, but there should have been. Campbell also left the game with a concussion following the hit.