I was listening to Real Crime Profile (a podcast with former profilers, and a casting director for some reason), and the ex-FBI profiler told a story about anxiety impacting the polygraph results. His brother was taking a polygraph (as part of a background check for employment, I think) and they asked him if he used marijuana, this in the old days when any marijuana usage, ANY usage, was an instant disqualifier. Someone in college once offered him some marijuana and he said no, and he got to thinking about how lucky he was that he said no, otherwise he wouldn't get the job. That made him so nervous that he was flunking the polygraph! Eventually the polygrapher asked what was wrong, brother told the story, and the polygrapher said "well, don't think about that next time I ask the question."
I'm not even sure how that suggestion worked - it's like telling someone not to think about their tongue in their mouth.