Even worse than that, there are in fact plenty of people who will systematically fail to show nervousness when lying, or more generally in socially-stressful situations, viz. psychopaths (in a broad sense, including Dark Triad traits more generally). As it happens, their lies are incredibly easy to detect for other reasons (basically, they never keep their stories straight, especially since they like to embellish them with all sorts of fanciful details), but still, it's hard to imagine a more catastrophic failure of this whole "lie detectors as infallible" paradigm.