When you gain several cards and play Monestary, the interface makes you choose all cards before trashing them. The correct way is that each is trashed in turn, so that any when-trash ability (like drawing cards from Overgrown Estate, Cultist, Rats) happens in between. See here: http://forum.dominionstrategy.com/index.php?topic=17897.0
just ran into tihs -- had two trashes with monestary, trashed a rats and drew a rats which it wouldn't let me trash.