It looks like there might be a bug in the way the number of tagged questions are calculated. If you look at a question (say, this one) and look at the number of questions tagged Perl, at this writing it says that there are 2889. However, if you click on the tag, it states that there are 2,891. Where did the two come from (or rather, where did they go?)
I followed up with Hellbog's suggestion, and decided to post a new question tagged 'perl' on SO. At that point, the number of tags displayed on that question increased to 2890. Refreshing the "tags" page, the number of 'questions tagged' there now stands at 2892. So, if it was a recalculation of different tables, it seems to have properly incremented on both. However, it seems silly to me to store that information in two places, and calculate it two times dynamically, as a new tag is added.
Could it be something else - perhaps the queries to calculate the numbers of tags are slightly different on the two pages?
Ok, cool. I'll swap this possible-bug. Before I do that, the list of bugs on the sidebar of this page and of the tag page at 817...
...and after, 'bug' sits at 817 on the sidebar and 816 on the search (one less on search, instead of more). Meanwhile, possible-bug lists as 67 on the sidebar, but 68 on the search. Ok, cool! Now I understand: on adding or removing tags during an edit, the 'tag count' doesn't change.
So, if that count can change when submitting a question, but doesn't change during the edit, is that still considered a bug?