J.B. McCloughan wrote on Apr 28^th, 2002 at 11:10pm:

It is hard to prove or disprove how many 'spies' have been 'caught' by the polygraph.  Polygraph is rarely referred to in-depth in the minuscule amount of publicized espionage cases, for obvious national security reasons.

Well, the most informed source one could hope for, ex-CIA Director John Deutch said something quite different.

Edward Curran also confirmed (see same thread as above) in a televised interview that no spy has ever been caught with a polygraph interrogation.

So, when comparing the veracity and accuracy between anonymous posters on the Internet who's livelihood depends upon deceiving others, and the words of an ex CIA Director and the head of the CIA's counterintelligence efforts , I think I'll choose the latter two.