[juliob]

This just happened to my Aussie friend, although not to such a degrading degree. I don't think either event qualifies as a false positive.

In either case, there was no evidence at all of an overstay. That's like a spam filter looking at your inbox and arbitrarily picking one email and putting it in your spam folder; would you call that a false positive? To have a false positive, there must be some sort of algorithm/procedure/heuristic, i.e. some sort of non-random selection mechanism. Otherwise, you don't have a spam filter, you have spam roulette. And that's how it is at the border with these immigration officers.