"Some experts extrapolate far beyond what can be supported," reads a 2009 National Academy of Sciences report about bloodstain pattern analysis. It adds, "The uncertainties associated with bloodstain-pattern analysis are enormous."
Many people have reservations when they hear criminal defendants claim to be innocent even though they pled guilty. Some can't imagine anything that could convince them to plead guilty to a crime they didn't commit.
According to the National Registry of Exonerations, at least 139 people were exonerated of crimes in the U.S. last year. Some 2,100 have been exonerated since 1989, when DNA evidence was first used to exonerate someone. Today, people are being found innocent using a variety of methods that don't always involve DNA, but the principle remains the same: Sometimes, the criminal justice system convicts innocent people.