This Court has never held that the Constitution forbids the execution of a convicted defendant who has had a full and fair trial but is later able to convince a habeas court that he is "actually" innocent. Quite to the contrary, we have repeatedly left that question unresolved, while expressing considerable doubt that any claim based on alleged "actual innocence" is constitutionally cognizable.
That's U.S. Supreme Court Associate Justice Antonin Scalia regarding Troy Anthony Davis who is on death row and is innocent.
Seven witnesses against Troy have since recanted.
Fair means where witnesses don't lie. There was no fair trial.