You've never heard of a "False Positive"? Women get false positives tests a lot usually the one that tells them they have Cervical Cancer (my girlfriend had one of those tests and it was a false positive)

A false positive obviously means that the test comes back positive but an error made it that way and so it is a bad test. Every testing thingy doctors use have X ammount of false positives per 100 or 1000 times they do it