I took a CBP polygraph a few weeks ago and walked out on it. All of the research I did predicted exactly what would happen, right down to the "calibration" questions and procedures. After the examiner attempted to get me to go over the same line of questioning a third time (claiming first that i was moving too much, then that my "mind was racing, and must be hiding something from her) while becoming increasingly hostile and accusatory, I ended the exam because I saw exactly where it was going and have no regrets about it.