will one lie on a rel question fail you?
any "reaction" on the test be it from a lie or just a reaction to the question for any number of reasons will result in a fail. Going into it with the intent of lying will probably not help your chances, since you will most likely have a reaction to that question since you are already concerned about it.
Actually, you are secretly expected to lie, or at least be less than completely truthful, when answering some of the questions -- the so-called "control" questions. Perversely, the more candidly one answers these questions, and as a consequence feels less anxiety when answering them, the more likely one is to fail. See The Lie Behind the Lie Detector for more on polygraph procedure:
http://antipolygraph.org/lie-behind-the-lie-detector.pdf