illegal formation?

[wisref2]

See casebook play 3.7.3 Situation A - AS CORRECTED BY THE NATIONAL FEDERATION (SEE THEIR WEBSITE)

(I've removed some of the situation in the play below to make is simpler to read)

B11 mistakenly believes he is his team's 12th player and leaves the field before the snap....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 (corrected) The activity is legal, but if done intentionally to gain an advantage it would be an unsportsmanlike foul. If B11 returns to the field after the snap it a live ball foul for illegal participation.