This expression can refer to the following: (1) When the hero(ine) in a horror movie thinks the worst has been and gone, but realizes (s)he's wrong. (2) When you're someplace with someone you once shared a past with; and then the situation suddenly tilts because of a word or a look, and you discover there's actually a whole other dimension to it...the past has become the present. Maybe even the future. Because it's not over, not yet.