American Football Rules IFAF Approved Ruling 3-5-3:V-VI

V. Team A is in formation to kick a field goal and Team B has eleven players in its formation. Just before the ball is snapped a twelfth Team B player runs onto the field. The ball is snapped and the kicker completes his kick. RULING: Live-ball foul. Five-yard penalty, previous
spot, or Team A may accept the result of the play. Since Team B’s twelfth player had not been in the formation when the ball was snapped, officials should not shut down the play for a dead-ball foul. The intent of Rule 3-5-3-b is to give Team B an opportunity to adjust its defense, but Team B must not be allowed to gain an advantage by an extra player entering the field very close to the time the ball is snapped. [Cited by 3-5-3-c]
VI. Team A lines up for a two-point try attempt at the B-3. Team B has eleven players in its defensive formation. A twelfth Team B player runs onto the field just before or just as the ball is snapped. A22 takes the handoff from the quarterback and (a) is tackled at the one- yard line; (b) carries the ball into the end zone. RULING: Live-ball foul by Team B for too many players on the field (Rule 1-1-1). Officials should not shut the play down. (a) Penalise Team B half the distance to the goal and repeat the try at the B-1.5. (b) Team A will decline the penalty on the successful try. [Cited by 3-5-3-c]