Funny how that last post is brought up just now. I just got home from a job where my Fluke 1AC failed me. You know how you are supposed to test it on a known live circuit before every time you really need it? Do you ALWAYS do that? I know I always used to. Then there are times where you are in an attic and there are no known lives near you, so you just trust it. I've also learn to simulate a live circuit (don't know how reliable it is but...) by rubbing the tester along my arm and it chirps just like being along a live conductor. Over the years complacency happens, and you just trust it more and more.
The problem with that is how those testers work, and if everything is not absolutely identical between the know live source and the unknown source, that only tells you that the device itself is functional, but does not eliminate the possibility of a false negative.