It maybe the lateness of the hour, but I've just had another one of those awkward thoughts that only people who know more than I do can provide an answer for (I'm compelled to edit quickly and note that by this I mean that noone I know's going to have a clue, not that I'm somehow an authority on the subject... you'll probably figure that out for yourselves). Anyway. Why doesn't the interference pattern, or perhaps more accurately, the evidence of an electron collision appear everywhere in the room the experiment is being conducted in? On the backside of the slit, on the front. On the walls. And so on... Would it?