If you get angry, it will "measure" that too, as a lie. Or if you get embarrassed, that "measures" as a lie. My grandma would have failed any cheater polygraph test horribly, as she turned red every time anybody mentioned anything related to sex in general. You could have asked her if somebody she never heard of had extramarital sex, and her answer, no matter what she said, would have showed up as a lie every time.
Physical or psychological exhaustion change the "results", too, of course. Or if you sit next to a window and the test takes long enough so that you start when you still sit in the shades but later the sun comes around and you sit in the light, that can be enough, too.
Or if you deliberately clench your anal sphincter, that's enough to change results. And yes, that actually works; that's why some lie detectors now feature ass-clenching detention technology.