If witnesses conspire against someone who really is guilty, (unknown to them) they are still punished. However, if they conspire against a murderer claiming to a court that he is a fugitive from another court, who convicted him as a murderer already, (and this truly happened, but unknown to the witnesses) then these witnesses are not punished.
(Q) Principle:
(A) In the first scenario, the conspiring witnessess were attempting to have a man executed, who was not yet sentenced to death by the court. (And some say that he could have been spared execution by confessing) However, once the defendant has been sentenced, as in the second scenario, it’s as though he’s already dead. And so the witnesses weren’t going to do to him something that wasn’t going to happen to him anyway.
*Connection: Mishna related, and same author as previous Sugya*