Someone created user32.dll the other day. Being a dutiful member of the New Tag Deletionist Cabal, I dove in to try and see if there could be a better, existing tag for the question, only to be greeted with nothing. The server returned 404, a Date header, and a content length of zero.
This is a known behavior with IIS that has bit SO before.
Given that this is known IIS behavior, the tagging system should make an effort to block users from creating new tags that will result in the site breaking.
(As for user32.dll, I was able to find the question and replace the tag with user32)