a) You are behind and the Silver pile is low
b) You are leading and the Curse pile is low
c) Your hand is Farming Village, KC,KC,KC,KC and your deck does not have any Treasures. And some actions you would like to KC. Edit: Extre,me case: hand: KCKCKCFarming Village. Deck: 3 Bridges.
d) You want the last Province for the win, your hand is Adventurer,Silver,3xcrap, and your deck has exactly 2 Golds.