When resolving a spell or ability that causes you to discard any number of cards you can end up discarding more cards than are in your hand. For example, your opponent can play Desecrated Earth targeting a land you control even if you have no cards in hand. When this spell resolves the targeted land will be destroyed, but nothing else will happen.

However, you cannot discard a card to pay a cost if you do not have the necessary number of cards to discard. For example, you cannot use Wild Mongrel‘s or Psychatog‘s first ability if you do not have any cards in hand.