nkf story - the lie detector

The Lie Detector I was reading the story of the one eyed dragon and how he told the court that he killed his victim by accident, misfired! Firing 6 shots by accident. Now what has this got to do in this thread? I thought everyone called to the witness box will have to swear to tell the truth, the whole truth and nothing but the truth? Looks like it is the other way round, to tell the lie, the whole lie and nothing but the lie. And then let the human lie detector to detect who is telling a bigger lie. There are two problems. The person telling lies can be a damn great liar that his lies become truth. The second problem is that the lie detector may wake up from the wrong side of the bed and misfunction. Justice is so scary.