One would guess you said that because you believe that no guilty person could pass a polygraph. Is that a correct assumption?
Several posters found examples of what you referred to. Why don't you respond back?
It makes no difference, really, about the polygraph. The reality of the situation is that they are like divining rods. They may or may not indicate where water is and are only as good as the belief of the rod holder.
Even if every suspect that was guilty passed a polygraph test, it proves nothing.
It does, however, raise more REASONABLE DOUBT. It probably bothers those that believe the Polygraph exam he was given is further proof he is guilty.
It really isn't. That is why it is not accepted in a court of law.
If this were the case, it would prove that polygraphs are a complete waste of time.
However, since it is the case that the guilty usually fail and the innocent usually pass, they do have some merit.
They are allowed in court for the defense as evidence of reasonable doubt, they are not allowed for the prosecution as evidence of guilt. A good system, IMO.
And LE are allowed to use them to help them eliminate suspects, (they do NOT eliminate by them alone, however)
All in all, a pretty good tool, in conjunction with other techniques.