That abnormally long and thick rogue hair that appears on your **** (*insert chin, neck, cheek, shoulder, nose, etc) only a day after you checked last. Most commonly complained about by women, and most commonly pointed out by a third party when you are least likely to have tweezers handy. Don't even both trying to pluck them with your fingernails...they're virtually invincible.
Sitting in a cab on the on the way to a formal event, the victim's most critical friend looks over and notices an "eyelash" on said victim's chin. With a delicate brush of her finger, critical friend attempts to dust it away, only to be met with spiny resistance. With a maniacal laugh and inability to hide her pleasure, critical friend declares "OMG, you've got a Mystery Hair!". The victim's face pales as she realizes there are no tweezers in her tiny, formal clutch.
(aside: Victim obsessively touches it throughout the evening, only to wake the next morning with the mystery hair twice as long and a small patch of acne).