Small castle says "Trash this or another castle from your hand. If you do, gain a castle"
So if I play Kings' Court on a Small Castle (no other castles in hand) then I should:
1. first time I trash small castle, and gain a castle
2. second time I don't trash a castle, and don't gain a castle
3. third time I don't trash a castle, and don't gain a castle
Instead, on each of the 3 times, I gained a castle.
Game #472 on frankfurt-test.
I think this bug actually applies to all the effects that depend on a trash (e.g. Mining Village has the same bug)