DNA tests are usually 99.99 percent accurate, which means they can be wrong 0.01 percent of the time. While those odds appear good, they are the accuracy rate under ideal scenarios where the integrity of samples is not a concern.
Human error is always a factor in the accuracy of any medical test, and DNA tests are no exception. Medical tests require humans handling the samples and humans interpreting the results. In experienced, trustworthy labs, the risk of human error is minimized, but it can never be eliminated completely. In less-experienced and less-trustworthy labs, there is a greater risk of human error.