What's a face mask penalty? A face mask penalty is exactly what it sounds like. It happens when a player grabs and twists or pulls down on another player's face mask, while making a tackle like in the image below. It results in a pretty severe 15-yard penalty, because it is so dangerous.
Before 2008, the NFL used to give out another type of less severe 5-yard penalty, if a player accidentally grabbed and released another player's face mask. Now, incidental grabbing of the face mask is not penalized, because the NFL wants to encourage players to quickly release in the event of an accidental face mask grab.