Of course, should you attempt to tag a repository without having a fully-updated working directory, Mercurial will fail with some nonsense about .hgtags having already been modified.
Ancillary discovery: should you typo when tagging a repository, the best way to recover appears to be to blow away your local repository and working directory and start again with a freshly-cloned one. Attempting to revert .hgtags doesn't appear to do anything.
Extra special discovery: it is impossible to tag a subset of a repository. Consequently, it is impossible to apply the same tag to different revisions of subsets of a repository. This is because you don't tag a file, or even a set of files - you tag a specific changeset.