4&10 A-30, punt team A enters the field, also Team B make his subs.
B has 12 on field and don't notice (well, actually 10; it's a 9x9 game)
Coach A notice it and yell to his center to snap the ball to get the penalty, but team A has not set one second
So:
Whistle dead ball at snap and ...
a) False Start (Illegal Shift) Team A
b) Illegal Substitution Team B
c) Offsetting fouls
I decided option c when this happened.
Thanks.