At first, i thought about buying a nine times embargoed card, but with perfect shuffle luck, he would still be able to buy a province. Mint is excluded, so he can't alter his next hand directly. But what if his opponent can now?
Maybe he bought a masquerade. His opponent has, a smugglers, a watchtower, a goons and three king's courts (6 cards, so maybe a caravan was played last turn), and let's say just copper and estates on his draw pile so his opponent couldn't have done anything to stop him otherwise.
By buying the masquerade, his opponent can play KC KC KC smugglers (gaining one masquerade, revealing the watchtower to put it on top of the deck) watchtower (drawing the masquerade), and then the famous goons masquerade pin kicks in. Or, I don't know what kind of deck he has, take out goons and just pass him three estates that have been drawn, so only two cards of his initial hand remain.
Edit: Screw that, see the following post. Sorry for totally ignoring this condition.