I've noticed this happen a few times. I've been sure that I've hidden a thread only for it to appear when I AS. I then hide the thread again only for it once again to appear when I AS something 
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Am I missing something here? I thought that when you hid a thread it was supposed to stay hidden until you decided to unhide it.
Generally if I've decided to hide a thread it's because I don't want to see it and I'm a bit when they then show up in my searches.