"I've heard that, but all of the cases I'm aware of where the person failed one they really were guilty and where they passed them they really were innocent."
I know of two cases where polygraphs were utterly wrong. There was a case back in the 80's where a man's sister and mother, who was rich, were killed. The man, who was a natural suspect, flunked not one, but two polygraphs. Turned out he had nothing to do with it.
In the other case, a woman went to the police to report she'd been kidnapped and raped. She also told them that her attacker videotaped the assault. The cops gave her a lie detector test - they did that to all rape victims - which she flunked, and they not only refused to investigate her assault, but threatened to arrest her for filing a false police report. Guess what? She was telling the truth, which they discovered after two more women were kidnapped and raped in the same way. When they found the rapist, they also found the videotape he'd made of the first woman's rape.
The fundamental problem with lie detectors is that they measure, not lies, but physiological reactions, the same kind of physiological reactions that anyone who's under emotional stress - like a rape victim or a guy whose family has just been killed - is likely to display.
All I know is that if I was innocent of a crime, I sure as hell wouldn't my life to depend on a polygraph.
Names in these cases, please. News cites would be helpful, too.