And that's not even the purpose of a polygraph - to prove if someone is lying or telling the truth. It's an interrogation tool to get someone to come out with the truth.
I had a polygraph. AFOSI conducted it. They knew I worked in an intelligence center, overseas, during Desert Storm several years before; which mean I had contact with foreign nationals, both through my work and off duty. I got though all the questions except the one dealing with giving classified information to foreign nationals. This went on for 3 days. They would come back after "analyzing" the results and told me there was a problem with that question. Then they would further interrogate me. When the analyst couldn't get me to fess up (because there was nothing to fess up to), he brought out his "supervisor"; a 6'20" red headed guy. That's when the interrogations got ugly. He yelled at me, spit in my face, berated me, etc... It was very nerve racking. I still denied any mishandling of classified. When I came back the next day, they said "you're good to go".
They were just trying to get me to confess to something. That's why when I see these so-called 'lie detector tests' I laugh that they are being used to determine whether someone is lying or telling the truth.