Individual servers can now pass back information to the router if they t...

...ry to serve the wrong page type

These are 'internal' and therefore cannot be hit directly. You can only hit them via the X-Accel method http://wiki.nginx.org/X-accel

The idea is that we have a small number amount of content whose urls do not pattern match properly. There are some articles whose URLs make them look like tag pages. 

by simply setting a header on the 404 we can bounce them to the correct proxy.