Innocence and the death penalty

The Death Penalty Information Center has a very informative website well worth the visit for anyone curious about the exactitude with which death sentences are meted out. By their count, since 1973 there have been 122 people in 25 states released from death row with evidence of their innocence. Florida leads the way with 21. Illinois is not far behind with 18. With statistics like these, can it be seriously argued that no innocent person has ever been executed? (See Power Line suggestion to this effect.) Again I ask, what is an acceptable rate of error in death penalty cases?