"Experienced" is a very subjective term; I have hauled the bodies of quite a few "experienced" outdoors folk out of the woods. The deal is that the first things affected by hypothermia, and by other types of environmental stress like hyperthermia, dehydration, etc., are the higher types of intellectual activity - making judgements, evaluating, and assessing conditions. just when you need to be really sharp and on the ball, you are impaired. I have gone down that slippery slope, fortunately not too far, a couple of times and "insidious" barely describes the situation.