Short answer, It is NOT a misdeal. Under TDA rule 27, a misdeal occurs in Flop Games ONLY if the first or second card dealt is exposed. Under Roberts Rule of Poker, v.11, Section 3, General Rules of Poker:
"MISDEALS
1. Once action begins, a misdeal cannot be called. The deal will be played, and no money will be returned to any player whose hand is fouled. In button games, action is considered to occur when two players after the blinds have acted on their hands. In stud games, action is considered to occur when two players after the forced bet have acted on their hands.
2. The following circumstances cause a misdeal, provided attention is called to the error before two players have acted on their hands.
(a) The first or second card of the hand has been exposed by a dealer error.
(b) Two or more cards have been exposed by the dealer.
(c) Two or more boxed cards (improperly faced cards) are found.
(d) Two or more extra cards have been dealt in the starting hands of a game.
(e) An incorrect number of cards has been dealt to a player, except the top card may be dealt if it goes to the player in proper sequence.
(f) Any card has been dealt out of the proper sequence (except an exposed card may be replaced by the burncard).
(g) The button was out of position.
(h) The first card was dealt to the wrong position.
(i) Cards have been dealt to an empty seat or a player not entitled to a hand.
(j) A player has been dealt out who is entitled to a hand. This player must be present at the table or have posted a blind or ante."
Unfortunately, I see nothing that would apply to rule a misdeal. In this case, my ruling would be that the UTG hand is dead, the
Button player is eligible to win 800K, the other 200K is returned to the UTG player.
btw: this is a similar situation from the 2009 WSOP, where (I think it was Katja Thater (sp)) who was all in with pocket Aces. At the showdown, her hand had been erroneously mucked by the dealer. The TD ruled (correctly, in my opinion) that her hand was dead and that she was eliminated.
Hope this helps!