The "Phantom" of Heilbronn and Negative Controls (Or The Lack Thereof) (scienceblogs.com, via Bruce Schneier's CryptoGram)
Summary: for four years, German authorities have been pursuing a mystery woman, suspected in six murders and numerous thefts extending over fifteen years. The only common piece of evidence they had about the woman is DNA evidence recovered at the scene.
Except that ... recently, German authorities admitted that the DNA is probably that of a factory worker at the plant that prepares the cotton swabs used for collecting DNA samples at crime scenes.
This is why science is hard to do. There are protocols that allow one to control for such activities, but you have to know that you need to do them.