[Xylakant]

And other problems with sampling: There’s the famous case of the “Phantom von Heilbronn”, where the same DNA was found on 40 unrelated crime scenes in multiple countries. In the end, it was found to belong to a worker in the factory that made the swabs https://en.m.wikipedia.org/wiki/Phantom_of_Heilbronn

[Gibbon1]

There was hopeless alcoholic in San Hoser who was arrested for murdering a businessman in Palo Alto. Turned out the same paramedic that helped take him to the hospital earlier in the day also was called to the scene of the murder. Some how DNA from hopeless drunk guy ended up under the fingernails of the victim.