You have a Microsoft SharePoint Server 2010 or Microsoft SharePoint Foundation 2010 web site and users use multiple Urls to access the same SharePoint site. You have configured the Urls that the users use to access the site via Alternate Access Mappings. In this scenario, users may experience intermittent unauthorized errors from the IIS web site (401) that is hosting the SharePoint site when accessing the site.
This problem occurs when an Alternate Access Mapping URL has been configured for the web application for a particular zone but the web application has not been extended into the respective zone. This creates a situation where multiple zone Urls end up pointing to the same backend IIS site, which can cause intermittent problems for users accessing the Urls.
To resolve this problem, remove the manually created public URL and extend the web application into the desired zone with the appropriate public URL and host header settings. This will automatically create a public URL in the alternate access mapping collection of the web application which the users can then use to access the site.