Have we killed an innocent man? I don't think anyone knows yet but considering the fact that we know we've had innocent men on death row, how can one escape the possibility that such a tragedy might have already occurred?
Yes, there have been people exonerated post-mortem. For example Cameron Todd Willingham, which I've mentioned a couple of times in this thread. Other probable innocents executed include:
-Carlos DeLuna, Texas, Conviction: 1983, Executed: 1989.
Evidence after his execution pointed at someone else as the probable killer.
- Ruben Cantu, Texas, Convicted: 1985, Executed: 1993.
Considered innocent after it turns out that the key witness was coaxed to the police to testify against him, and had to be showed Cantu's picture 3 times before he could actually point him out in a lineout.
- Larry Griffin, Missouri. Conviction: 1981, Executed: 1995.
Both a survivor of the shootout for which he was accused, and the first policemen on the scene actually said it wasn't him. Convicted and executed anyway. Based on that and new evidence pointing at someone else, he's considered one of the most categoric examples of an executed innocent.
- Joseph O'Dell, Virginia, Conviction: 1986, Executed: 1997
He actually asked repeatedly for a DNA analysis to prove his innocence, and it was denied. It was done only after his execution, and, what do you know, the blood wasn't his after all. Pretty clear case of innocent, for lack of other evidence against him.
- David Spence, Texas, Conviction: 1984, Executed: 1997
The police lieutenant who supervised the investigation and a detective involved were already convinced he's innocent before it even went to trial. All witnesses against him were inmates who had been promised favours in return for testifying against him. No physical evidence or anything else connected him to the murders.
- Leo Jones, Florida, Convicted: 1981, Executed: 1998.
The confession was extracted under torture, he later tried to recant it but it just earned him the dirt nap. Both cops who interrogated him were discharged from the police. New evidence points at someone else.
- Gary Graham, Texas, Convicted: 1981, Executed: 2000
One witness claimed to identify him after having seen him briefly from a distance and through a window. Two others who saw the killer up close said it was _not_ Graham. For some reason (incompetence? malice?) Graham's lawyer did not use either of them as witnesses. The jury never even knew those two witnesses existed.
So, yes, we _do_ know. It already happened.