Negative information

What generations of students of bad teachers have known intuitively is now scientific fact: Information can be negative, i.e. you know less after receiving it than you knew before. But you needn't worry about this being some sort of brain virus snatching that laboriously collected education from under your eyes (or from behind there, actually), because we're talking about the wonderful world of quantum physics, of course. Here is a popularised description of the concepts, together with links to the tough stuff (via slashdot).