Illegal substitution?

[HLin NC]

If the 12th man goes BACK off and no other player came off, you've got illegal sub under 3-7-3 at least and under 9-6-7d could have illegal participation if "at or immediately before the snap" for using a pretended substitution to deceive. Of course, they is no definition of "immediately" beyond our old 3-5 second rule of thumb.

Case play 3-7-3 C deals with the formation question. The only question I see is had A gone into "formation?" If that 12 player that entered lined up as a receiver or a back, I believe he's bought the farm and somebody else better come out.