There is convincing evidence that Carlos DeLuna was executed for a murder that was actually committed by Carlos Hernandez:
The ultimate villain of this awful story, Hernandez died in prison, in 1999, boasting to the end that he had killed Wanda Lopez and allowed another man to take the fall for it. The cops knew this. The prosecutors knew or should have known it. Witnesses knew it. And yet no one did anything to stop the state executioners from carrying out their job. Why no one listened to Hernandez for all those years, and why no one hears the cries of others today, is a question Justice Scalia and many others have to answer for themselves.
Scalia is one of three people chided in the article, along with Ann Coulter and George W. Bush, for expressing "total confidence in the American justice system and, in particular, its ability to never, ever execute people who were not guilty." One mistake might not make the case against the death penalty, but it should at least discourage people from having such absolute certainty.
Every human endeavor is subjected to the possibility of error because human beings are flawed. What sets capital punishment apart is that its mistakes can never be undone. That's the essence of the libertarian argument against the death penalty, which does not hold that it's unfair or immoral, only that it is too much power to trust the state with.