Possible contamination from the lab is not a very reallistic reason. They are very careful and very accurate, after all, they have very extensive training, and they enjoy what they do, so they will take pride in it.
Yeah, right, in an ideal world.
Might be true for
most private labs maybe, probably not so much for government labs. And every security person working at airports is very careful and accurate.
How about that crematorium that buried all those bodies instead of cremating them?
There are always those that will take short cuts in order to make a few more $$.
There are humans running the tests. Mistakes can be made. When my daughter was born, they brought the wrong baby (a boy) to my wife to breast feed. and this was at one of the better hospitals.
If the results are in doubt, it's best get a diferent independent lab to double check. Just like people are encouraged to get second opinions on doctor assessments, if they don't feel that what they are being told is correct.