The coworker at my last job was a ex cop. I asked him a lot of questions about this shit. He said you can lie as much as you want on a polygraph. I said BS, he said: look if it was accurate, they would use it in court. I know the shit he did he passed just fine. I forgot which one exactly, but a few serial killers passed theirs just fine and they were 100% guilty and convicted.

You can also fail even if you are not guilty.

I looked into this quite a bit when I was considering a career in a LE department.