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In this question, I got 2 upvotes, and system has recognized it, as it comes in my reputation history. But my reputation score at the top of the screen doesn't change. Why? Is it a bug?
You've hit the reputation cap for today, and you won't receive any more reputation from up-votes until tomorrow.
You can see your current reputation for today on your reputation page. You've received 202 reputation so far: 200 from up-votes and 2 from accepting an answer.
If this is the first time you hit the cap you'll get the Mortarboard badge. In fact if you look at that page now (30th July 2011) you'll see your name at the top of the list.
Keep doing it and you'll eventually get the Epic and Legendary badges.