Consider the classical double slit experiment where we are shooting electrons trough a double slit and then capture them on a photographic plate. We know that the pattern of electrons on the plate will differ depending on the state of knowledge of the observer. If he make a measurement to find out witch slit the electron passed through, the interferens pattern will dissapear. This is a well known fact. Now consider a more complicated situation which have the above situation as a special case: Assume the condition is like before, we have the classical experiment with an observer. But now assume that this first observer (person 1) is contained in a box, so that no one outside the box will know whether the person in the box observes the electron passing through the slit or not. The person outside (person 2) will only see the photographic plate. Person 1 in the box will see the whole experiment including the photographic plate. Consider now the situation when person 1 make a measurement to figure out what slit the electron passed through. As a consequense of this the interferens pattern will dissapear and he is all satisfied with that. From person 2's perspective, he will not know which slit the electron passed through and so he should see an interferens pattern on the plate. But this contradicts the fact that person 1 did not see interference. One could argue that once person 2 outside the box sees the interference he will conclude that person 1 did an observation, and so he knows that person 1 knows what slit the electrons passed through. But that doesnt resolve the fact that person 2 didn't know at the time the electron passed through the slit whether person 1 made an observation or not. I don't know, but at first sight this situaion seems paradoxical!