I have a number of webapps running on an IIS server that need to use HTTPS. I force a redirect to the HTTPS version from HTTP using a custom error 404.3 configured in IIS, that loads a page that redirects to the HTTPS login page.
When I tested this in Firefox, Safari, and Chrome it worked fine, but then I discovered that IE was serving up its own error page simply saying access forbidden.
It turns out IE will ignore any custom error pages that are less than 512 bytes in size. The simple solution is to add some extra text in a comment to the HTML to pad it out, then it works ok.