The Phantom of Heilbronn, often alternatively referred to as the "Woman Without a Face", was a hypothesized unknown female serial killer whose existence was inferred from DNA evidence found at numerous crime scenes in Austria, France and Germany from 1993 to 2009.
The only connection between the crimes was DNA, which as of March 2009 had been recovered from 40 crime scenes, ranging from murders to burglaries. In late March 2009, investigators concluded that the "Phantom" criminal did not exist, and the DNA recovered at the crime scenes had already been present on the cotton swabs used for collecting DNA samples; they belonged to a woman who worked at the factory where they were made
tl;dr- a serial killer was thought to have existed because the DNA of a woman who worked at a cotton swab factory kept on being found at the scene