A cotton swab may be the most wanted criminal in Europe. For years, cops across southeastern Europe have hunted an cop-killing eastern European woman whose DNA turned up at 17 crime scenes. The crimes were wildly diverse, geographically separated, and had no visible pattern.
You see where this is going, right? It's now believed that the DNA was introduced to the forensic swabs at the factory, and that cops have been hunting someone who probably sticks q-tips in baggies all day and has never committed a crime.
It now turns out that the several-hundred-men task force might have really been chasing a phantom. Alarmed by the apparent randomness of the crimes, involving both highly professional work and seemingly amateur break-ins, they started checking for contaminations in the labwork. The likeliest suspect now are the cotton swabs used to collect evidence at the crime scene. All the swabs used in the forensics works were sourced from the same supplier, a company in northern Germany that employs several eastern-European women that would fit the profile. Even more incriminating, the state of Bavaria lies right in the center of the crimes' locations, without ever finding matching DNA in crimes on its territory. Guess what: they get their cotton swabs from a different supplier.
By the way: contaminated cotton swabs aren"t as trivial to avoid as one might think. It's relatively easy to sterilize cotton to prevent infections. Forensics however require a complete destruction or removal of any DNA contamination, which is apparently a lot harder.