For online play, the important part is what happens when a player gained one (or more) 'ungainable' cards and one (or more) gainables. According to Donald on the forum.dominionstrategy thread, choosing the ungainable one should be an option. This doesn't happen on dominion.games at the moment (if there is exactly one gainable card, this is gained automatically).
This seems to affect only Smugglers, because I think it is the only such card where the the decision what to copy depends on a choice right there. Other cards (like Mint), choose the card beforehand.