When the hair on the back of your neck stands on end !!!

Perhaps not quite so dramatic, but a friend of a friend once came by my house with a young man who made my every 6th sense alarm bell go off. He seemed polite enough, normal enough, etc., but I told that person to never bring that guy around again. Sure enough, two weeks later, the guy who had made my hair stand on end had some sort of psychotic break and tried to burn down his apartment with his two roommates in it. Luckily the authorities arrived in time to put out the fire and deport the dude to the local funny farm. I've had other such 6th sense warnings about people which have proven correct, so I pay a great deal of attention to my intuition on such things.