Some jobs in the defense sector require a lie-detector test (or polygraph) as part of their hiring process, even for interns. The Defense Intelligence Agency (DIA) is one such agency.
Let's consider a hypothetical situation where you left the military, used your GI Bill for a college degree, and have just been accepted to the DIA's intern program. Congratulations! It looks like a great program and I hear they pay well (for an internship). You start your application process, go in for the lie-detector test, and fail it.
For SHAME! Okay, that's an exaggeration, because failing the lie-detector test isn't actually that big of a deal.
Here are two reasons you may have failed:
You are a liar
The first reason you may have failed is that you are a liar and a cad. If this is you, I advise you do some major soul-searching and, until you discover yourself and change your ways, stop applying for government jobs that people with better values and judgment deserve.
You were nervous
At the risk of being naïve and idealistic, I'm going to assume that no veterans fall into the above category, which means you failed because you were nervous. You can fail the test simply because you don't quite understand the question, or over-analyze the question each time, even if the examiner gave you clarification multiple times.
Let's say, for example, the examiner asks a question about foreign contacts. They may have explained to you that this means foreign contacts that you have regular contact with, but maybe you studied abroad for a year and are still "friends" with people all over the social media, including people from other countries. For some reason, this fact keeps popping into your mind every time the question is asked. You tell the examiner, and they just say it's not something to worry about, that the question does not refer to them. But they ask you again, and again you fail that portion of the test. There's nothing you could have done differently.
What happens next
The most likely next step is that you will be invited back in for a second lie-detector test, and I hope you pass this one (if you were telling the truth the first time). The people who administer the lie-detector test know it isn't exactly accurate, so you get some leeway. However, remember that your hiring agency may be on a time-crunch and, if this does happen, you could possibly lose out on the internship (or full-time position).
Be ready for this hypothetical situation in case you find it happening to you, and if it does, don't worry about it. Focus instead on doing better next time, or meditating for a while before the test to make sure you're not nervous at all about any of the potential questions they may ask. Maybe the lie detector test will change in the future, but for now, don't sweat it.