A subtle form of
irony. In this case the
knowledge that comes to light is found to be
Unpleasant,
painful, or anything that makes the victim say
Fuck me gently with a chainsaw.
More often than not the situation involves romantic feelings of some sort. Repeated exposure to this sort of irony leads to bitterness and cynicism. See any emo kid for clarification, or many, many, many examples.