As of July 11, 2008, 129 people have been exonerated from US death rows since 1973. That’s an average of over 3 per year who have either been pardoned, had their charges dismissed, or been acquitted on retrial. Three men have been exonerated so far this year. Only 16 of these cases have involved DNA evidence (most murders that don’t involve rape do not have the kind of evidence that makes DNA testing useful).
Mistakes that had led to all these wrongful death sentences have included: inadequate legal representation at trial, police and prosecutorial misconduct, perjured testimony or mistaken eyewitness testimony, use of notoriously unreliable jailhouse "snitch" testimony, the suppression and/or misinterpretation of mitigating evidence, and community or political pressure to solve a case.
All these shortcomings still exist in our criminal justice system, so it is inevitable that we will continue to see wrongful convictions and wrongful death sentences, and maybe even wrongful executions. While it is nearly impossible to “prove” that someone who has been executed was innocent, due to the fact that evidence is usually discarded following the carrying out of a death sentence, there are several examples of cases where it is extremely likely that an innocent man was executed.
The death penalty is unnecessary for achieving justice – adequate alternatives exist. There is no need to continue risking the execution of an innocent person when we don’t have to.