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  1.  
    Currently, this question is tagged with "tag-removed" but I don't think it was before, and the edit history suggests that tag-removed was one of the original tags: http://mathoverflow.net/questions/18100/theorems-with-unexpected-conclusions

    My guess is that some vague tag was used, and this was merged with tag-removed, with the unexpected consequence that this question with 2 other tags, 32 up votes, and over 2k views is now incorrectly tagged.
  2.  
    One of the uses of [tag-removed] (the original use?) is as the default tag into which to merge tags that should not exist. A recent example which I know about from another thread is the "general" tag. This practice keeps the questions from being bumped, with the awkward side effect you've noticed. However, the moderators typically encourage users to removed [tag-removed] from questions that have other tags when they are at the top of the front page anyway -- or at least that's my impression.
  3.  

    Jonas got it exactly right. When moderators merge tags, questions don't get bumped, so it's a much less disruptive and more efficient way to eliminate unwanted tags (like [general], [newbie], or [math]). If you see a question on the homepage with the [tag-removed] tag, please remove it. The only time you should add the [tag-removed] tag is when a question doesn't belong on MO at all and there aren't any tags that fit it (the software requires that every question have at least one tag).

  4.  
    Alright, I removed [tag-removed].