Defensive Illegal Participation vs Illegal Substitution

[TxBJ]

Agree.  The other two likely will not know of he got off (or close enough).  Only the deep guy on that side needs to be focused on that.

[TXMike]

The rule is clear.  Unfortunately some conference philosophies have blurred things and caused confusion.  The situation you described is a live ball sub infraction foul.  An exception would be where the officials realize as the guy is leaving that he actually had been in the formation for more than 3 seconds but they had missed it until he started to leave.  Then they could "justify" shutting it down and treating as a dead ball foul. 

Here is a play example.  Defense subs, the replaced player is jogging off and ball is snapped.  The deep flank shut it down and they handled as a dead ball foul. 
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[Dakota Dan]

Do not forget that the HL/LJ could have a DOF if the 12th player trying to get off the field and he is on the wrong side of the LOS when the ball is snapped ...  So, you could have two flags down on that play, one for ILS and one for DOF... I have seen this a couple of times this year.

[NCAA-SJ]

Defense subs, the replaced player is jogging off and ball is snapped.  The deep flank shut it down and they handled as a dead ball foul. 
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Mike, I think the crew messed this up.  You have to ask yourself this...is the a substitution in process?  Meaning....the process is a sub comes on, communicates with a replaced player, replaced player leaves the field. 
If there is no process in effect (i.e. 12 players standing around in formation with no one trying to leave the field), then you have to go dead ball foul.  If this is not recognized until after the snap by the officials, then you are stuck, by the rule, with IP (I think most agree with this from what I read).  The first play (Boise game), if you don't shut it down (and I mean shut it down with guns ablazing), then you HAVE to go IP...no choice!!!  Ding for them.
If a 12th player is attempting to leave the field (i.e. process is in effect), then this is a live ball foul because the process is not completed before the snap...this CANNOT be shut down as, by rule, it is not a foul until the snap (no different than an illegal motion or shift by A--the snap creates the foul).

In short,
12 players standing in formation means no one leaves with 3 seconds=dead ball 5yd
12 players participate in play because SFB did not count in time=live ball 15yd
12 players on field with one 'trying' to get off field when BALL IS SNAPPED=live ball 5yd

[TXMike]

I agree. IMHO this should not have been shut down.  But I believe there are conferences who want it done just like this

[NCAA-SJ]

I think that may be because they do not realize they have a 5 yard live ball foul at their disposal.  Some think it's only 5 dead or 15 live...unfortunately.

[TXMike]

This was a Big-XII crew. I am pretty sure they know there is a 5 yard live ball foul available.

[NCAA-SJ]

I would tend to agree with you, but the video, unfortunately, does not seem to show that.  Unless, of course, this is just one official's mistake.

Either way, no sense beating that horse....God forbid, anyone start posting my oversights/errors/misjudgments/screw ups 

[Birddog]

This was a Big-XII crew. I am pretty sure they know there is a 5 yard live ball foul available.

Big 12 crew did exact same thing in an A&M game.  Defender is clearly retreating and near sideline when whistled dead for illegal sub.    This is how supervisor Anderson wants it I guess.

[TXMike]

Big 12 crew did exact same thing in an A&M game.  Defender is clearly retreating and near sideline when whistled dead for illegal sub. 

When was this?  I don't recall seeing it and just checked the box score and do not see an illegal sub foul noted.