When people are "prepared" for the polygraph (say a mole) they are often trained to construct alternative mental models that would let them answer damaging questions without appearing anxious.

For example, they might organize an obvious seemingly accidental meetup in a some monitored public place (train station) with a foreigner from a specific country so when the question is asked "have you met with foreigners from country X" they can say "yes, I sat next to one when taking the train".