Apple has confirmed that it’s possible for a FaceTime caller to listen to the person on the other end of the call — and even see them — before they pick up. Making use of this newly-discovered bug requires actions someone isn’t likely to do accidentally, which is probably why It wasn’t noticed during testing.
UPDATE: Apple said this evening it will quickly fix this serious privacy flaw. In the mean time, it has also disabled its servers needed for Group FaceTime to function.