Nice. Sounds like a riddle:
The Curse pile is empty. The only action card in your hand is Familiar. You play it, and your opponent winds up gaining a Curse. How can this be?
This has only one answer, as Masquerade doesn't cause your opponent to "gain" a Curse (for example, he can't gain a silver instead with Trader).
With multiple actions, your opponent could gain two Curses: Familiar draws Shanty Town, Shanty Town draws Ambassador and Familiar, Ambassador returns two Curses to the supply and your opponent gains one, and Familiar causes your opponent to gain another.