It happens all the time around here, especially on campus. I've seen it happen directly to friends of mine from other friends even. It's a bizarre phenomenon, but it's always happening.
There are reasons for the sterotyping, mostly ignorance. If average Joe white guy doesn't have any black friends and only knows about the bad side of black people as they're often portrayed in the media or something, well, through no fault of his own he's going to be a bit prejudice. That's not excusing that guy for being prejudics; it's terrible that there are people out there who simply go on what Dateline tells them, but that's the case.
It boils down like this: there's a lot of ignorant people in the world, and many of them are white and sheltered. That's why these things happen. It's sad and it's depressing to know that all those stereotypes exist despite the strides we've made as a society, but I think that things like this will always happen, because nowhere in the world is there a place where no one is prejucdice or racist. You just have to remind yourself that the idiots who walk across the street because they see a black guy are just that, idiots, who don't know any better, and unfortunately no one can change everyone's mind. I guess the best thing you can do is try to ignore it, remind yourself that they're in the minority of people out there, and laugh at the fact that you just walking down the street is enough to make some middle aged white guy soil his drawers
I don't mean to come off nonchalant about this or anything, it's just that I've noticed it for a long time and have found the best way to deal with such things is to laugh it off, because people aren't going to change, and they just end up looking dumb to those of us who understand that the world is a big and diverse place, not some pasty biodome. Hopefully you guys get what I'm saying.