On the main page, I upvoted a story and then opened the comment thread and I could upvote it again. It only worked on some stories. I then went to 'new' and tried it there, and there too it worked on some only. I haven't tried the other way around (first upvote from comment thread and then from the list of stories). Is this a bug?
The http request for the first vote was probably still being processed when the second page was generated, so it still produced the up-arrow. When you clicked on it, though, the second vote would have been ignored by the server.