Since 1993, a particular woman's DNA has been found at a series of seemingly unrelated crime scenes in Germany, France and Austria. These have ranged from carjackings to burgularies and have included the scenes of six unsolved murders, the most notorious of which was the killing of a young police officer in Heilbronn Germany. An enormous international police task force has been chasing this "Phantom of Heilbronn" for years and a 300,000 euro reward has been offered to no avail. Analysis of markers in her DNA indicated that she was of eastern European origin and it was even speculated from mitochondrial DNA analysis that she might have a masculine appearance.
But wait. The most likely explanation for all of this: contamination of the DNA collection swabs at the factory in northern Germany where they are manufactured. The "Phantom of Heilbronn" is probably just an assembly-line swab-stuffer whose DNA got into the product. It seems that, until very recently, no one ever did the simple control experiment of running a swab right out of the package...