Police agencies were convinced that they had a serial killer committing multiple murders due to the DNA found at the crime scenes. It was considered quite unusual, however, that the suspect was a woman.
Only later was it discovered that the woman whose DNA was found worked for the company that made the products which were used to collect DNA samples. Somehow the products had become contaminated with this woman's DNA.
Yep. wiki on that case. Police were using cotton swabs which had not been certified for DNA collection. I'm going to guess the woman whose DNA was found on the swabs handled the swabs with her bare hands at some point.