A "bug" is essentially when a program fails to function as expected/desired.
What I'm looking for is a word for the opposite of that - a function that is desired but isn't supposed to be working yet unexpectedly starts working. As an example, I'm developing an iOS app at the moment. It includes notifications upon certain events. We didn't think anyone had deployed our push notification server yet and notifications hadn't been tested at all, but in the course of testing other things, notifications somehow started showing up.
I feel like this is a feeling familiar to all programmers. Is there a word?