A man takes pictures of his children on a camping trip. They're naked. His life becomes "a living hell".
Positive predictive value is a very subtle concept , but somehow we need to teach it far more widely. In the meanwhile, be very careful about the children's pictures.
 In this case the "test" is the judgment of the clerk at the photography store. Given that the prior probability of pornography in family photographs publicly developed is probably much less than 1%, even if the clerks have spectacularly good judgment the vast majority of "positive" results will in fact be false positives. In the world of medicine, this test would not be FDA approved.