When you gain something from the trash with Graverobber, and choose to exchange it for a Changeling, the card in the trash goes to its supply pile. I don't get why and think it should stay in the trash.
And when you gain something from the trash without a supply pile (like the Zombies), you don't get the option to exchange it for changeling at all.
This is all working as intended by Donald X.
http://forum.dominionstrategy.com/index.php?topic=17991.msg736073#msg736073
And the paragraph from the Nocturne rulebook explaining how "exchange" works:
"Nocturne has three cards that tell a player to "exchange" a card for another card. The card being
exchanged is returned to its Supply pile, or non-Supply pile, and the card being exchanged for is taken
and put into the player's discard pile. This does not count as gaining a card. The exchange only happens if
both cards can be exchanged; if the pile is empty, the cards are not exchanged."
Thanks for the clarification. This was unexpected for me.