Valid for Sitecore 6.x
  Known issues in Sitecore CMS 6 when IIS is configured to work in Integrated Pipeline mode.

Reference Number: 320886.

This page contains a known issue associated with Sitecore CMS 6 when IIS is configured to work in Integrated Pipeline mode. When denying read access to the home item of the site and accessing it with a short URL, such as http://mysite.com, Sitecore may redirect to the "layout not found" page instead of the site's login page or the "no access" page.
Workaround:
Deny read access to the item that is specified by the "rootPath" attribute of the current site.