Our network admin recently noticed some unusual errors in the proxy logs: www.rottnestisland.com/en/pages was failing to redirect to the default page. Fiddlering the request revealed a 401 response was being returned by SharePoint and friends.
Naturally our first inclination was to scratch our heads and wonder why such a thing would occur; at first glance both /en and /en/pages share many similarities with regular IIS directories; shouldn't SharePoint just go looking for the default page?!? After all, a request for /en is successfully 302 Redirected to /en/pages/home.aspx and the authenticated site also redirects the /en/pages request.
In the above case, home.aspx is the default page as configured in the Pages list and it is published along with its master page and other supporting resources (in fact the page loads without issue when requested directly).
Our anonymous setup isn't out of the ordinary: a read/write web app running in the default security zone is extended to a second read-only web app running in the internet zone with anonymous access enabled. Most annoyingly, we've got some URL rewriting tricks going on with www.westernaustralia.com so that site (which runs in the same farm as Rottnest) does redirect the /pages directory, as does its sister web site, www.tourism.wa.gov.au… the latter of which should have no tricks applied!
Until this point we assumed those of us in the naughty developer team had broken something on the Rottnest site and those of its sibling sites running the same code base. When I spun up a sanity web app in my dev machine, however, I found I could reproduce the problem in that environment with no customisations applied.
From one point of view this all makes sense: subsites will 302 Redirect to the default page in the Pages library and behave in the "normal IIS way"; the Pages library, being a list, should not do the same and does not—it's not a site. While plausible, this argument falls down when everything just works in the authenticated site.
No deeper understanding or solution to this one as yet…