Personal Foul causing fumble

Question

In the NFL, supposing a player catches the ball legally and runs with it, for a total of a 25 yard gain, but is hit to the head, causing a fumble before he makes contact with the ground (and is knocked out cold).

The opposing team lands on the loose ball and the refs whistle. I'm pretty sure the ruling would be the foul is assessed at the end of the run before losing the ball, thus the offensive team would have gained a total of 40 yards on that play.

However, the play was not dead at that spot (all other fouls would have nullified the play if the opposing team accepted the foul, so how can you nullify just half a play?). My question is, where would the ball be spotted and is there a reference that points to this scenario?

Answer 1

This scenario in the rule book pertains to your situation. Basically the penalty is enforced from the spot of the foul and the offense keeps the ball with a couple exceptions:

FOUL ENFORCEMENT ON RUNNING PLAY WITH POSSESSION CHANGE
Article 13: When a defensive foul occurs during a running play (3-27-2) and the run in which the foul occurs
is followed by a change of possession, the spot of enforcement is the spot of the foul and ball reverts to
offensive team. See 14-1-12- Exc. 5.
Exceptions:
(1) When the spot of a foul is in advance of the spot where the offensive player lost possession, the spot
of enforcement is the spot where player possession was lost and the ball reverts to offensive team.
(2) When the spot of a foul by the defense is at, behind, or beyond the line of scrimmage, and such foul
incurs a penalty that results in the offensive team being short of the line, the ball will be advanced to
the previous spot.

Answer 2

According to the NFL Rulebook, Rule 8, Article 7 (page 53 of the 2011 Rulebook), a the enforcement in your example would be from the spot of the fumble, as it was forward of the line of scrimmage. Had the fumble or penalty occurred behind the line of scrimmage, it would've been from the previous spot.

ENFORCEMENT SPOT DURING A BACKWARD PASS OR FUMBLE
Article 7 Enforcement Spot During a Backward Pass or Fumble. When a foul occurs during a 
backward pass or fumble, the basic spot of enforcement is the spot of the backward pass or fumble. 
 Exceptions: 
(a) Behind the Line. When the spot of the backward pass or fumble is behind the line (including in A’s end 
zone), if either team fouls, the spot of enforcement is the previous spot, even if B’s foul is in A’s end 
zone. If Team A fouls in its own end zone during a backward pass or fumble, the enforcement of the 
penalty results in a safety

The rule for a penalty during a forward pass is slightly different, but has the same result: enforcement is from either the previous spot or the dead ball spot, whichever favors the offense more.