Straight from the NFL rulebook.. no mention of "face guarding" or similar... but the 4th quarter bogus PI penalty is explained nicely in the "Actions that do not constitute pass interference include but are not limited to" section.
http://www.nfl.com/rulebook/passinterference
Pass Interference
There shall be no interference with a forward pass thrown from behind the line. The restriction for the passing team starts with the snap. The restriction on the defensive team starts when the ball leaves the passer’s hand. Both restrictions end when the ball is touched by anyone.
The penalty for defensive pass interference is an automatic first down at the spot of the foul. If interference is in the end zone, it is first down for the offense on the defense’s 1-yard line. If previous spot was inside the defense’s 1-yard line, penalty is half the distance to the goal line.
The penalty for offensive pass interference is 10 yards from the previous spot.
It is pass interference by either team when any player movement beyond the line of scrimmage significantly hinders the progress of an eligible player of such player’s opportunity to catch the ball. Offensive pass interference rules apply from the time the ball is snapped until the ball is touched. Defensive pass interference rules apply from the time the ball is thrown until the ball is touched.
Actions that constitute defensive pass interference include but are not limited to:
(a) Contact by a defender who is not playing the ball and such contact restricts the receiver’s opportunity to make the catch.
(b) Playing through the back of a receiver in an attempt to make a play on the ball.
(c) Grabbing a receiver’s arm(s) in such a manner that restricts his opportunity to catch a pass.
(d) Extending an arm across the body of a receiver thus restricting his ability to catch a pass, regardless of whether the defender is playing the ball.
(e) Cutting off the path of a receiver by making contact with him without playing the ball.
(f) Hooking a receiver in an attempt to get to the ball in such a manner that it causes the receiver’s body to turn prior to the ball arriving.
Actions that do not constitute pass interference include but are not limited to:
(a) Incidental contact by a defender’s hands, arms, or body when both players are competing for the ball, or neither player is looking for the ball. If there is any question whether contact is incidental, the ruling shall be no interference.
(b) Inadvertent tangling of feet when both players are playing the ball or neither player is playing the ball.
(c) Contact that would normally be considered pass interference, but the pass is clearly uncatchable by the involved players.
(d) Laying a hand on a receiver that does not restrict the receiver in an attempt to make a play on the ball.
(e) Contact by a defender who has gained position on a receiver in an attempt to catch the ball.
Actions that constitute offensive pass interference include but are not limited to:
(a) Blocking downfield by an offensive player prior to the ball being touched.
(b) Initiating contact with a defender by shoving or pushing off thus creating a separation in an attempt to catch a pass.
(c) Driving through a defender who has established a position on the field.
Actions that do not constitute offensive pass interference include but are not limited to:
(a) Incidental contact by a receiver’s hands, arms, or body when both players are competing for the ball or neither player is looking for the ball.
(b) Inadvertent touching of feet when both players are playing the ball or neither player is playing the ball.
(c) Contact that would normally be considered pass interference, but the ball is clearly uncatchable by involved players.
Note 1: If there is any question whether player contact is incidental, the ruling should be no interference.
Note 2: Defensive players have as much right to the path of the ball as eligible offensive players.
Note 3: Pass interference for both teams ends when the pass is touched.
Note 4: There can be no pass interference at or behind the line of scrimmage, but defensive actions such as tackling a receiver can still result in a 5-yard penalty for defensive holding, if accepted.
Note 5: Whenever a team presents an apparent punting formation, defensive pass interference is not to be called for action on the end man on the line of scrimmage, or an eligible receiver behind the line of scrimmage who is aligned or in motion more than one yard outside the end man on the line. Defensive holding, such as tackling a receiver, still can be called and result in a 5-yard penalty and automatic first down from the previous spot, if accepted. Offensive pass interference rules still apply.
