One area which leads to the most mistakes is sample preparation. When the technician removes the sample to prepare it to go into the machine, there are numerous steps that must be done correctly. If the steps are not performed correctly, then the results are not reliable. And when the results are not reliable, they should be rejected.
One crucial step is the pipetting of the samples. A pipette is like a turkey baster which is used to draw fluids out of the containers submitted to the lab and transferred them to vials for testing. Put the wrong fluid in the container, or the wrong amount, and the test could be incorrectly identified or improperly elevated.
How you pipette does matter. If you do it wrong, your results are wrong. Not defendable, but just plain garbage. So, next time you have a case where pipetting occurs, log into the library and see whether they did it right.