There is no documented example of the polygraph ever catching a mole. On the contrary, all of the known moles who were subjected to polygraph screening beat the polygraph.
Saying that they "beat" the test gives the test too much credence. The test is a ploy to make the subject confess. The only step to beating the test, is the simple knowledge that it's a ploy in the first place.
Point well taken. But the U.S. government does consider the polygraph to be a pass/fail procedure. Applicants and employees suffer severe career harm for "failing."
Wasn't Philby caught by polygraph but argued his way out of the results? (He was, in fact, a spy all along).
The real problem is when this hocus pocus enters the legal system and people get convicted based on examiner testimony or face the presumed guilt applied to anyone who refuses a polygraph.