IN 2008, GERMAN detectives were on the trail of the "Phantom of Heilbronn." A serial killer and thief, the Phantom murdered immigrants and a cop, robbed a gemstone trader, and munched on a cookie while burglarizing a caravan. Police mobilized across borders, offered a large reward, and racked up more than 16,000 hours on the hunt. But they struggled to discern a pattern to the crimes, other than the DNA profile the Phantom left at 40 crime scenes in Germany, France, and Austria.
At long last, they found the Phantom: An elderly Polish worker in a factory that produced the swabs police used to collect DNA. She had somehow contaminated the swabs as she worked. Crime scene investigators had, in turn, contaminated dozens of crime scenes with her DNA.