flop gone bad

[knifeandfork]

10/20 limit O/8 9 handed
5 players see a flop of JcTc9s
the sb bets, utg mucks and the 6d mysteriously shows up in the flop under the Tc
utg cards did not contain the 6d and this was clear. the 6d was stuck to the back of the Tc on the flop.
the action was normal up until the point that the flop was disturbed by utg mucking and the 6d revealed.
how do you proceed from here ? Does the fact that there was a bet and a fold on what appeared to be a perfectly well dealt flop change anything? i believe if the error is immediately detected when the cards come out they all get shuffled back and a new flop dealt but there has been action on this flop and a player has mucked his cards? thanks, and sorry if this has been asked and answered many times but it was the first time i ever saw the card stuck and remain undetected.
jason