On math.SE a tag called (arithmetic--topology) has been created a few days ago. (It was created on Jun 9 at 10:19 according to the list of new tags.) The two hyphens in the tag name are probably a typo an the tag name is going to be corrected eventually. But still this revealed an interesting bug.
If you display the questions having this tag, you see "0 questions tagged (arithmetic--topology)". The popup says "0 followers, 1 question". As you can check here, there is a question having this tag.
Since the tag has been there for a few days, I doubt that this is caused by caching. Could the reason be the unusual tag name containing "--"?