Got scheduled for a poly test

[jdoe994]

will one lie on a rel  question fail you?

[notguilty1]

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.

[George W. Maschke]

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