A couple of years ago I was on a Virgin 747 over New York - everyone was eating lunch, I was looking out the window where if I pressed my face against the window, I had a huge field of view. Being an aircraft nerd, I was watching all the other aircraft above & below us crossing in the air corridors in that really busy airspace.
Suddenly, whoooosh - a white 737 went past us in the opposite direction, exactly the same altitude, so close that if it wasn't slightly banked, our wingtips would have clipped - so close I could clearly see all the rivets on the plane, the faces in the windows, the position of the control surfaces etc - looking at the B52 photo above, the 737 would have been between the B52 and the window of the photograher.
I got up and quietly told the stewerdess who contacted the captain - who replied that the other aircraft had been 'at least a mile away'.
I have the image clearly in my head and it still bothers me whenever I fly.