I have been doing a lot of shopping lately (much more than my wife likes) and I have witnessed a number of times where people ask a harried worker in a crowded store who is invariably doing something else if they have any more "item x." They always say no.

This is a perfect (potential) false negative scenario, and is also the same feeling you would get if a penetration test resulted in no compromise. I never get any comfort whatsoever from an answer like that (and generally don’t ask anyway).