The interface for marking a question as a "favorite" displays somewhat suprising behavior. Say I open a question in one tab in my browser, and also happen to have the same question open in a different tab. (It's easy to do this unknowingly if one's clicking on a bunch of "related question" or "possible duplicate" links.) Say that the question in question starts out not marked as a favorite.
First oddity is that when I hover over the greyed-out favorites star, the pop-up message says "This is a favorite question (click again to undo)". Isn't this a bug? The question has not yet been marked as a favorite in any tab.
Second oddity: suppose I do now mark the question as a favorite in tab1. Then I go to tab2. (I think it doesn't matter whether tab1 is closed at this point, or later.) Tab2 will not show the question as a favorite, even if I wait a while. OK, that's fine. But now suppose I do click on the favorites star in tab2. That star will now light up. Looks like the question is now marked as a favorite. However if I then close all my tabs, then return to the question, it will be marked as not a favorite. Apparently the server is interpreting my click on the favorites button as a toggle, and when I click in the second tab, the +favorite status I had set in tab1 gets changed to -favorites. Even though tab2 displays the question as favorited.
Using Firefox 8.0.1 on FreeBSD.