Judging a Book by it's cover

There is a very famous saying "Don't judge a book by its cover". I'm sure like most people, I've been guilty more than a few times. It's easier for me to pick a book off of the shelf when I see a cover that interests me or a title that has a certain "melody" to it.

I've often wondered if it's like that with people. I've met quite a few people and certainly judged them by their cover. Yes I know that we shouldn't do that, but we're only human after all. I don't mean I judged their looks or their clothes, but their personalities. Time and time again, I get a nice little "HA!" from God when my closest friends turn out to be the ones that I didn't quite get along with at first... OR it would be the other way around. Now don't get me wrong, I'm not saying all situations turn out this way, just some. People often tell me that their first impression of me is either:

1. Very happy-go-lucky, cheerful, energetic, people person. Someone that seems very sweet but also talks a lot, giggles a lot, and seems eager.

2. Quiet, shy, polite, and doesn't seem unless spoken to. Doesn't seem like a people person and doesn't like to mingle.

I find this funny because these two assumptions are very different from one another. Which would mean that judging a book by its cover has big flaws. I would say that my personality is not quite like 1 but not quite like 2 either. I'm maybe in between or way apart from those two assumptions. What about you guys? Did you ever feel like you've judged someone by their "cover" but turned out to be wrong?