What can cause a sample to fail when tested for water quality?

Samples can fail for many reasons. Samples are taken by a Sanitarian from the health department. They are not to be collected when it's raining or when there are heavy winds so that nothing compromises the sample. Did they collect the sample from a contaminated water tap because of animals or insects. Finally did they run the tests correctly. Positive samples are always rechecked immediately at that site as well as one upstream & one site downstream. We have never had a follow-up test came back bad ever.