There can never be a 100% accurate lie detector, only a 100% "thinks they're telling the truth detector". Ultimately human memory is deeply flawed and we're capable of having entirely false memories and swearing on our lives to things that never occurred. A machine which can perfectly read our brains can only get this messy imperfect mix.

Even in the sense of political intrigue, is it so hard to imagine someone so brainwashed they truly believe the lie they are telling you?