The National Coaliton to Abolish the Death Penalty notes that 112 people under the death penalty have been released from prison as not just undeserving of the ultimate sanction, but substantively innocent, since 1972. That is roughly the equivalent of randomly ordering that one company—let's just say Company C, 100th Battalion, 442d Regimental Combat Team—be shot. Supposedly, this will improve discipline in the rest of the army.
The problem is not the death penalty per se; the problem is that our system of justice is not good enough to always convict only the guilty, let alone assess the death penalty only to those who "deserve" it, even leaving aside the issue of the criteria for "deserve." Although intellectually I'd like to believe that one could construct such a system, I no longer do. I do not believe that even the best-case adversarial system, with an absolutely level playing field between prosecution and defense counsel, with a police force consisting only of absolutely unbiased and truthful and perfect judges, with definitive and absolutely reliable forensic evidence—in short, Plato's Republic (remembering that in his search for justice, he never found it…)—would be accurate enough. It is not just the difficulty of adequate maintenance for the machinery of death; it is that we can only find out about errors retrospectively, and can do little or nothing for the defendant with a death wish.