On the "Recent changes" page. Mediawiki displays "Hide anonymous users" and "Hide registered users" options. It's all good for public wikis, but for private wikis or wikis requiring users to be registered to make modifications, theses options are not useful and not pertinent.
Though MediaWiki now uses active filters on the Recent Changes pages, users can still disable the improved version of this page and be presented with this little UI bug. It is also possible that MediaWiki offers the Unregistered or Registered filters on private/non-public wiki, but I have not been able to check since the MediaWiki version I use is too old.