I see another thread (Frilled Mystic does not counter Unstopable Slasher)...adding to this that it does not counter anything(?) I have had it twice now in a game just as described in the other post be cast in response to an opponent's spell and, with those being the only things on the stack, their spell is not countered and resolves after I confirm that I'd like to counter the spell. The log I'm supplying here is with a game against W Ajani Cats when some 3-mana Cat-lord was cast...and was not countered.