I think this picture sums it up. I am a person who is a very different person inside than I display on the outside. Therefore I shouldn't be surprised when someone at work asked me what I was listening to on my iPod. I told them (Massive Attack, Prodigy, Muse, Kovaks, Sneaker Pimps, IAMX and so on) they were taken aback. "I didn't think you would be the sort of person that would be into that music....you are a dark horse, aren't you?"
Yup - don't judge a book etc. We can all be guilty of this on some level; it is borne out of our animalistic side trying to figure whether the person in their orbit is a threat or not. Maybe we 'positively' pigeonhole until we are proved to the contrary, automatically scanning for those who fit into our tribe, so to speak. As a mature adult, I assess and try not to pigeonhole too quickly.