DNA is not foolproof. Why? Because the testing isn't always done correctly. William Thompson is one of the leading skeptics--It's not like CSI in real life, mistakes are made, you can't always trust a positive match.

In March, a 21-year-old Texas man serving a 25-year sentence for a 1998 rape was released from prison after Thompson's review of DNA evidence in the case. Prosecutors argued at trial that Josiah Sutton's DNA was a 1-in-694,000 match with the rapist's. It took a jury just two hours to convict him. In fact, Thompson was able to show that Sutton's DNA wasn't found in any of the police samples.

None of this surprised Thompson, an attorney who also has a doctoral degree in psychology.

Although cases of innocent people falsely convicted by bad DNA evidence are rare, Thompson sees ambiguous results, sloppy lab work or overstated findings in a quarter of the cases he examines.