Goons/Watchtower game, and both of you have buys to spare and are buying up Copper for points so the pile is low. If your opponent depletes Copper on his turn, you lose. You calculate that he has 7 buys possible from his deck. You buy Copper until there are 8 left, and 2 Curses exactly so that he can't empty that pile either(but you need exactly those 2 points to win). Having 8+ buys yourself, you will definitely be able to win next turn.