How about this one?

[bigjohn]

What if the B player returns from his own sideline?

3.7.3 SITUATION A: B11 mistakenly believes he is his team's 12th player and
leaves the field before the snap (a) on his opponent's sideline, or (b) on his sideline
and enters his team box. B11 then discovers his error and returns to field on
his team's side of the neutral zone before the snap. RULING: In (a) B11 has committed
illegal substitution by leaving on the opponent’s sideline. (3-7-2) In (b),
there is no foul as long as B11 remains a player. (2-32-1) If done intentionally to
gain an advantage, it is an illegal participation foul or it could be an unsportsmanlike
foul. If B11 returns to the field after the snap in either (a) or (b) and participates,
it is a live-ball foul for illegal participation. (9-5-1f; 9-6-4 a and d)