I took one as part of a job interview; it was standard proceedure. I told the truth, but that damned machine said I was lying. It really sucks losing a job because of a mechanical false witness.
Since then, I have realized that there are TWO ways to beat a polygraph: by lying and by telling the truth. There is a reason polygraphs are not allowed in a court of law: they are faulty.
You see, just as extremely dishonest people, with no conscience, can lie and make it think they are telling the truth, extremely honest people, with a strong conscience, can tell the truth and make it think they are lying. This is due to a stronger internal system of checks and balances.
It appears that, in an honest person, anything they have had problems with in the past, even though they have been conquored and corrected long ago, still snags an honest person up in a polygraph test. Heck, as a kid, I was a little pyro-maniac. Now, I have a healthy respect for fire and don't even like having a bottle of alcohol in my house. BUT, if you put me on a polygraph and ask me about some recent fire somewhere (even if it were impossible for me to have been there), that machine would say I'm lying, and it would then be wrongly assumed that I "must have done it" (then I'd get arrested).
People's blind, ignorant acceptance that polygraph tests are accurate has led to a police state where they can and will charge and arrest you on the testimony of a faulty machine. To make it worse, there is a general blind assumption that, by refusing to take a polygraph test, "you must be guilty of something". WRONG.
Never take a polygraph test at the request of any law enforcment. agency. Never. They often abuse that faulty machine and use it to charge innocent people and jail them. They can not force you to take one, nor is it any kind of admission of guilt to refuse one (they will try to make you think it is). Even if you have done nothing, they will jail you and mess up your life and record forever.
People's blind assumption that polygraph tests are true has also led to a flood of notable false witnesses on TV. Among them, you will find (the notorious) Maurey Povitch, John Walsh and Nancy Grace (that former JUDGE should know better). People like this, who take such a flimsy testimony as fact, have no real interest in the TRUTH at all. They are more interested in sensationalism and such; truth is irrelevent.
Anyone who takes the word of a polygraph test is a liar and false witness.
"These six things doth the LORD hate: yea, seven are an abomination unto him: A proud look, a lying tongue, and hands that shed innocent blood, An heart that deviseth wicked imaginations, feet that be swift in running to mischief, A false witness that speaketh lies, and he that soweth discord among brethren". Proverbs 6:16-19
"Thou shalt not bear false witness against thy neighbour". Exodus 20:16 (remember that one?)