Then there's the famous case of Dr. Lee at Los Alamos. First they gave him a polygraph that showed he was telling the truth. Then they decided he had engaged in espianage, so they went back and looked at the polygraph results again, and decided that they showed that he had lied. In the end, it turned out that he hadn't engaged in espianage.

Basically, the polygraph is a Rorschach test for the polygraph examiner.