From the National Institute of Health's EMBO report June 2010:
There is also considerable risk arising from basic laboratory errors, as the hunt for the elusive phantom of Heilbronn' demonstrated. For years, German, French and Austrian police investigators pursued a female suspect who, according to DNA evidence, had committed dozens of crimes and several murders over the course of 14 years including shooting two police officers in Heilbronn in 2007. Eventually, however, they discovered that the DNA profile retrieved from 40 crime scenes in three countries actually came from an employee of the Austrian company that produced the swabs police used to take samples at the crime scenes.
When it comes to detective work, improvements in protocols combined with the ability to obtain partial profiles from small and degraded samples are having a huge impact, according to Reinhard Schmid, head of Biometric Identification at the Austrian Federal Ministry of the Interior. Particular progress has been made with previously unsolvable cold cases' in which DNA samples now provide clues to the suspect's identity that can be followed up by traditional detective work. The cold case hit rate obtained with DNA has reached 93%, Schmid said. That means that without using DNA analyses and the DNA database, 93% of those cold crimes could not have been solved because there was no link to a possible offender after the crime had been committed. In each case DNA analyses provided objective evidenceoften leading to [the] release of innocent suspects.
The very fact that it is possible to obtain DNA profiles from degraded samples means that there are borderline cases in which the accuracy of DNA profiles is doubtful
But care must be taken, cautioned Ate Kloosterman, Professor of Biometrics at the University of Amsterdam and senior forensic scientist at the Netherlands Forensic Institute. The DNA samples used to thaw cold cases are derived from samples taken at the time, often not based on the latest protocols to collect evidence without contamination from crime scenes.
And let's not forget that *touch* DNA is even trickier to deal with when it comes to crime-solving.