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".
"so you stuffed up a bit, stop cryin 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's glaring at you from across the room. She's just spilt milk anyway.