None

I can imagine they would have to release him. If you try to kill someone and get it wrong you can't just keep trying to kill them, because that would amount to cruel and unusual punishment. Especially since in reality a failed lethal injection would probably cause major and very painful medical problems. Nor could you just keep someone in prison after their sentence had been carried out because that would be arbitrary justice. Nor could you try to re-sentence them because that would be double jeopardy.