A case in Germany illustrates how easy it can be for contamination to sneak into labs from unexpected sources:
Police in Germany have admitted that a woman they have been hunting for more than 15 years never in fact existed.
She was alternatively called the "woman without a face" and the "phantom of Heilbronn" after the city in southern Germany where she allegedly killed a policewoman.
Police suspicions were based on traces of identical female DNA they found at 40 crime scenes across southern Germany and Austria.
Now it has been determined that the cotton swabs used to collect DNA had been contaminated accidently by a woman working at an unidentified factory in Bavaria.
The company selling the swabs did not claim they were DNA-free or certified for use in DNA analysis.