That isn’t how the tests are performed. Periodically a test will generate a positive where it hasn’t in the past for a lot of different reasons. False positives are inherent to the system, just as false negatives are.
There should probably be a better system to dispute positive samples. I can see how that would make you bitter at the process.
I tested positive for a controlled narcotic. I take a beta blocker. How could you get a false positive from that?