illegal participation

[mbyron]

3.7.5 SITUATION: Substitute (a) A1, or (b) B1, noticing his team has only 10
players on the field, comes onto the field just as the ball is about to be snapped.
RULING: In (a), A1 must be on the field on A’s side of the neutral zone, inside the
9-yard marks, and not violate the shift or motion provisions. Furthermore, the act
of his coming onto the field must not deceive the defensive team. In (b), the substitution
is legal as long as B1 is on the field on B’s side of the neutral zone prior
to the snap. (3-7-6; 7-2-1,6,7; 9-6-4)

The case does not discuss the penalty: if the covering official judges the foul to be accidental or negligent, it's a 5 yard illegal formation penalty for not being within the 9-yard marks prior to the down (7-2-1). If the official judges that the offense is deliberately deceiving the defense, it's unsportsmanlike conduct (9-5-1(f)).