You have just played an Action card from your hand, but have yet to carry out the action described on the card. You decide for whatever reason you don't feel like doing it, so you don't. Your opponent suspects you may be cheating and demands to see your hand. You argue that you are not required to show your hand.
As a matter of fact, both you and your opponent are correct; you are in fact cheating, yet you are not technically required to show your hand. Which cards could you have just played?
Solved for one example: Throne Room.