The NFL occasionally puts together videos to educate officials, players, the media and the public about what constitutes an penalty. One day, this Janoris Jenkins hit on Fred Davis could well appear on one of those videos.
It has all the elements of a 15-yard penalty for an illegal hit on a defenseless receiver. Davis has the ball but he has not had time to make a football move. Jenkins launches into him and hits him above the neck. That's exactly what the rule book says should be a personal foul.
Only problem was that the officials did not throw a flag. Fortunately it seems that Davis is OK.
(h/t SBNation for the GIF)