Old news from 2007 but this past year the police interrogations and polygraph of convicted murder Teresa Kohnle have been posted online. Here is her polygraph at the 33:09 mark:

https://www.youtube.com/watch?v=SOO0IrgoZok&t=33m9s

What I take from this is that this lady could have gotten away with the murder if she kept her mouth shut.  Her first mistake was speaking with police without a lawyer.  Even during the polygraph, before which they make her sign a "stipulation agreement" which was a big mistake, she does a good job of sticking to her story about not knowing how the fire started and could have pinned the fire on her dead husband.   

After the polygraph, the polygrapher and police give her the standard post-test interrogation trying to get her to confess, but she sticks to her guns.  They never show her the polygraph charts and the whole polygraph test may have just been a formality to get to the post-test interrogation part where the police and polygrapher go in for the kill.  She wasn't arrested and convicted until after the police interviewed her kids who confirmed they saw black smoke after their mom went back into the house.  She may have actually beaten the polygraph!

Takeaway:  Polygraphs are just used to get you to make a confession.  Never take one for criminal investigation, and if you are taking one for a law enforcement of intelligence job, stick to your story and don't make a disqualifying admission.