My quick solutions:
1. Your opponent has fewer than three cards in hand.
2. You have already played possession and b-crat lets you hurt the hand your opponent will actually play.
3. Your opponent has a hand of harems (e.g. harem x4, curse)
4. Your opponent has a hand of Council room x3, throne room, nobles.
5. Your opponent has a remodel in hand with curse x3, fairgrounds.
6. Your opponent has a single harem, a mint, curse x3.
7. Your opponent has a copy of the only victory card in hand already on top of his deck and has harvest in hand.
8. Your opponent has a hand with HoP, Great Hall, silver, silver, silver with havens giving 5 drawless cards to play.
9. You are in a 3er and you know the other opponent will swindle his province -> peddler.
10. Your opponent has an apprentice engine capable of hitting 2 colonies and has curse x3, apprentice, colony in hand.
11. Your opponent has a minion in hand and will just go for four anyways (the GS will let him keep the green in hand and pick up treasure.
12. Your opponent has menage, estate x2, silver x2 in hand.
13. Your opponent has curse x2, province x2, lookout in hand.
14. Your opponent has tunnel and in hand discarder (e.g. cellar, secret chamber).
15. Your opponent has SP in hand and you want to have better odds of it not drawing his whole deck.
15. Your opponent has HP, silver x3, estate in hand.
16. Your opponent has a Pstone in hand that increases in value if you GS, but not if you b-crat.