Moral of the story: If you get an interview, be really nice to Jessica.
Bad idea.
I can't tell for sure, but what Jessica is most likely interpreting is sub-conscious gestural leakage. A fancy term meaning you exhibit both facial and gestural signs that can be read. For example if you are be probed to check for flexibility, a necessity in "fog of war" situations in Startups, and you exhibit a "fearful" or "sad" facial micro-expression ~ http://face.paulekman.com/aboutmett2.aspx this could be how Jessica is determining a flake. You can tell a person maybe showing certain emotions but you cannot be sure if they are "conscious" (lying) or "unconscious". Judgment is required. Conscious lying can also be masked by certain types of pharmaceuticals which can mask these feelings.
YCombinator could improve themselves in the selection process by getting better at recognising the fundamentals of faces, emotion and deception ~ http://www.paulekman.com/micros/ Naturals such as Jessica interpret but not fully understand the science of it meaning you can get false positives.