A good quote from the FAQ:
As unbelievable as this may sound, even federal agencies like the FBI and Secret Service assume that all of their applicants have cheated in school, lied to people in authority, driven under the influence of alcohol, etc.
Q. You maintain that polygraphs are biased against the truthful. How can this be?
A. Applicants who explain all their past indiscretions to the polygrapher during the “pre-test” interview and actually give honest answers to the “control” questions (and, as a consequence, feel less anxiety when answering them) face an increased likelihood of “failing” the “test.”
EDIT: Good! Wait… they had that power?!