Something mildly bad/unpleasant which has already happened, and nothing much can be done about. Usually referred to in the context of "stop crying over spilt milk".
Something unimportant, especially the former significant other of the person you're now dating. Refers particularly to the casualties resulting from boyfriend/girlfriend stealing.
Oh, don't worry that Jen'sglaring at you from across the room. She's just spilt milk anyway.