illegal substitution?
 

A11 thinking he has been replaced leaves the field and realizes he is in the game. he then re-enters and meets all the requirements. is this a penalty?

Not if no other player came in for him.
This is what I like about Reddings Study Guide. It has an explanation to help explain situations like this;
"A player who returns to his team box, realizes he was supposed to stay in the game, and then returns to the field does not commit a foul because he does not lose his status when he enters team area while the ball is dead. He is not a replaced player because he was not notified by a substitute that he was to leave the field and he is not a substitute because there was not a player vacancy for him to fill."