We got a ruling from Jerry Diehl on the casebook play 10.4.5 Sit J. that we spent a lot of time discussing.
This is the one that has B1 intercept the ball in the EZ, fumble it, it rolls OB at the 2. Meanwhile B2 has committed a foul at the 9 after the change of possession.
The casebook says to the end of the run is the 2 contradicting another casebook play. Jerry says this is incorrect. The end of the run (and the basic spot) is the spot of the fumble in the EZ. Resulting in a safety. So unless the FED comes up with an exception, this is treated just the same as any other play.
BTW, I understand this play went around the country and there were up to 4 different interpretations.

I'm not Fed, but this question raises another. I don't mean to hijack the thread - but I think this applies, at least to understand the logic.

If, in Fed, a ball is intercepted in the endzone, and while that player is running, but still in the endzone, another member of his team blocks in the back or holds in the endzone, do you have a safety? If not - it sounds like your interp is wrong... If so... why?!?! This seems wrong - the reasoning behind making a foul in the endzone on a "normal" play a safety is that the foul potentially prevented an ACTUAL safety. This would not be the case where an interception occurred in the endzone, and such reasoning would not make sense. It seems to me it should be marked off from the 20 (or declined if the runner came out and didn't reach the 10.)

Fed rules have a specific exception: "When the final result is a touchback," then we penalize from the succeeding spot (i.e. the 20). On the fumble play, since it went back into the field of play, we don't have a touchback anymore, and we're forced to fall back on the general rule, giving us the safety. Whether or not this is "fair," it is the rule.

Yes, you have a safety... If the runner is downed in the end zone, the basic spot is the 20... But the foul by the offense occurred behind the basic spot, so the penalty is enforced from the spot of the foul... Since the spot of the foul is in the EZ, it's a safety...

Yes, it is a bit unfair, but them's the rules... To emphasize the unfairness, consider this play:

1/10@A15... On a draw play, A33 busts off an 85 yard TD run... During the run, the OT and DE end up in A's end zone, where A70 facemasks B58... The result of the play is a TD, the ball was at the B40 when the foul occurred, but the foul is enforced at the spot of the foul, resulting in a safety...

Wouldn't force be an issue in this situation? Since B did not put the ball into the endzone, how can you penalize them for intercepting it there? I would think if the penalty is accepted, you'd have a touchback and mark it off from the 20....

Since A is responsible for the ball entering the EZ, it becomes a touchback. Since the foul was at the 9, not in the EZ, first and ten at the ten.

Rule 8-5.2(c) states, It is a safety when....A player on offense commits a foul for which the penalty is accepted and enforcement is from a spot in his end zone;

It is a loose ball play, the enforcement of the penalty is the end of the run, which is the EZ.

Therefore you have a safety.