Every test has false positives and false negatives. These can cause many problems.
Many years ago I was doing graduate research in a government atomic research lab and asked for some samples to be analyzed. I got such weird results that I talked to the flame-ionization detector technician and learned she had been there about five years and never checked her standards. The lab director shrugged his shoulders and said he had to work with what he had. I made up my own standard and re-calibrated the machine on a weekend, ran my own samples, and never told anyone about it. I’ve been somewhat suspicious of lab results since.