I have taken a polygraph, after my house was involved in a home invasion. I was the victim, treated as the perp, answered every question 100% honestly, and was told I "failed miserably". I had absolutely no knowledge, or involvement with the home invasion, was 100% a victim. This was in Brevard County Florida.
I did get an apology from them when a few weeks later they caught some one doing a home invasion elsewhere in the county, who confessed to a string of them, my house being one.
But in short, I can see why they are not used in court (unless both defense and prosecution agree) - imho, they are worthless.
When I took mine - I was in the room with the interviewer just talking with him (unhooked) about the home invasion, and he went over the exact questions he was going to ask three or four times with me, almost so I could formulate an answer for the real test.
I was told to lie to a particular question. He held up a card with the number 4 on it and asked if the card had the number 4, and I was told to lie and say no. Other standard questions were asked, such as is John Robert Doe your name is 1/1/60 your birthdate, etc. I was also asked questions that I felt had no bearing on the home invasion, and questions that were very ambigous. I had the tester clarify some questions, such as "Have you ever taken anything of value form some one that loved or trusted you?" "Ever? Well, I took a dollar out of my mom's purse when I was 9, does that count?"
There were questions that I never really understood. The incident happened when I was 48, and the questioner prefaced a lot of questions with "prior to your 46th birthday did you ever....".
I was then hooked up to the machine. All in all I was asked about 50 total questions, with several of the questions repeated more than twice during each session. I was officially asked the sets of questions three times.
The questioner afterwards told me I passed all questions except one where it showed deceit when asked if I felt responsible for the home invansion - explaining that was natural, because since I was insured, it was a victimless crime, did I in some way help them, etc - at the time I didn't realize it but in retrospect, I think I passed those questions too, and he was trying to trick me into a confession. I was never shown a copy of the polygraph, or a copy of his evaluation of it, but on a copy of the final police report, the investigating detective said I had showed "extremely high levels of deciet, and not only failed, but failed miserably.