IMHO, this would be a

that never should have flown

. If an official said to the coach that the re-entry was OK to the coach, then the official erred, not the coach. If the opposing coach challenged this, a response could be : "Coach, in error we told him it was OK, this was our error, not his. If we errored in responding to your question, we would have reacted the same." Things to remember about this rule :
(1) An injured player can return for the next play IF the injury occurred on the last play of the 2nd or 4th quarter;
(2) While a team can't 'buy' a TO to keep an injured player in, if a team calls a TO before an official spots an injury the player can remain in;
(3) A dead ball foul (false start, etc) doesn't count as a play;
(4) If a player shows signs of a possible concussion, he cannot re-enter until approved by a health-care-professional.