I was to take a polygraph exam to become an animal control officer for the Fairfax County Police Department. 
Let me say first of all, the employment process to become any type of officer is not based on experience, education, or anything that would actually be important to performing the position you apply for. I have 10+ years of experience in the animal field, but apparently relevant experience is not valued or even considered. Becoming an officer is entirely based on being able to recall long forgotten and irrelevant information about your past in great detail, then being able to remember exactly how you answered that question previously, then answering it exactly the same every time. 
Now you may think that if you have nothing to hide, this wouldn't be too difficult. Unless you have lived an incredibly uneventful life, you'd be wrong. It is very difficult, at least it was for me, to recall when and how many times certain things occurred for me in my life. It isn't like I documented how many times I smoked pot, fought with my boyfriend, and got a parking ticket. But these are the things they ask you. 
So, the first step in the employment process is filling out a 30+ page "personal history statement" which is essentially a document that asks you about every little detail of your life, and most importantly every single thing you may have done wrong in your life. When I filled mine out, I did the best I could, but some things needed to be approximated, because I just didn't know. My advice to anyone filling this out.. be honest completely, but more importantly REMEMBER exactly how you answered these questions, because this is entirely what they compare your polygraph answers to. Honestly, I think you could lie on your PHS but as long as your lies are consistent throughout, you would probably do better than I did. 
So, here's when I ran into trouble. When I filled out the PHS I did not purposely omit or falsify any information except my drug history. I knew I could pass a drug test currently, and so I figured they could not prove anything if I lied. Now, I wish I had just been honest, because apparently they don't care if you use drugs, they just want to know about it? Supposedly. I assumed that a drug history was admitting to a crime and was most likely automatic grounds for being disqualified, and so I was afraid to admit drug use. So I lied on the PHS, then guess what? I felt guilty, and especially when I got in the room with the examiner I felt like I would be better off coming clean, since they supposedly value honesty so much, right? 
Ok so the day of my polygraph, first off I went to the wrong address. I had gotten 2 emails with the wrong address and one with a new address that I didn't specifically remember getting, and it was sent about a month before the exam. So the night before when I was figuring out where to go I looked at the wrong one. I ended up being late to the exam, which started me and the examiner off on the wrong foot. When I explained what happened he didn't believe me. He literally said "How about we start with the truth?" "Why were you really late this morning?" Then proceeded to tell me that I received this email with the new address on the 5th at a certain time, and I opened it at a certain time. And I simply said, "Well, then I messed up. I didn't remember receiving it, and I looked at the wrong email, I'm sorry." Super great start. Also, I'm being treated like a criminal immediately, not a candidate for employment, which is important to note I think. I felt like a criminal from the very first step of this entire process, because they really only care about catching you in lies and knowing about your transgressions. 
So before the polygraph begins, the examiner asks you questions, rapid fire, about everything you may have done wrong in your life. Most of this info was on the PHS, but also more personal stuff. He also asks you about your employment history, drug use, criminal history, and ultimately this entire process is to poke holes in what you said on your PHS. Basically, if you deviate at all, or provide details not provided on your PHS, or cannot accurately remember an event or timeline, it is assumed you are lying about it. For instance, I couldn't really remember the last year of college, and my overlapping bartending job timelines for that time period. So he hounded me about that for about 15 minutes. I wasn't trying to lie about anything, the time period and series of events was just fuzzy for me, and was about 7 years ago. 
He asked if there was anything on my PHS I wanted to come clean about, which is when I admitted to my past drug use,(btw, I have never been a regular drug user, but I have smoked pot like most people) and he asked lots of questions pertaining to that, which I answered to the best of my abilities. The questions he asked went much deeper than the PHS, so of course there are going to be things that come up that weren't on there, which seemed to be used as an opportunity to catch you in a lie.
Ultimately, my honesty did not pay off. After "consulting with a supervisor" it was determined that I had too many "discrepancies" between my PHS and what I was saying. Mind you I had filled out the PHS a good 2 months before this as well, adding to not remembering exactly how I answered everything. I never was hooked up to the polygraph machine, so I never even got the chance to possibly "prove" I was being honest. 
Basically, I found this entire process to be demeaning, criminalizing, and I didn't feel as if I was being evaluated on my current ability to perform a job, but my ability to submit myself to judgment and harassment. I left feeling like I had done something wrong. I left feeling generally icky and shitty, like I was in trouble, like I was a bad person, which I know I am not. This type of process for employment is absolutely ridiculous, and in my belief doesn't weed out candidates based on any real or relevant metrics. 
The takeaway message is be honest and consistent and hope for the best. If you didn't put something on your PHS, just don't mention it or bring it up because it won't help you. 
Good luck, kids!