This is just a guess, but it seems plausible: My bet is that this is related to [this accident](https://meta.superuser.com/a/1135/561) in which all [tag:windows-xp] tags were destroyed. Note that this happened *after* quack edited the question for the last time before you did yesterday.

If something was wrong with the fix for the accident (the question had been closed at some point, maybe that's related as well), it may have happened that the question's rendered tags (which is what's usually displayed) and the corresponding last revision (shown [here](http://superuser.com/posts/110258/revisions)) are out of sync. The last revision is also what's used in the tag editor, since it's technically the most correct tag list (even if in this case, it wasn't really).

Since your edit fixed this and this was very likely the result of an age-old issue, I'll mark this [meta-tag:status-completed] for lack of a "status-hmm-yeah" tag.