Took me a while to solve this, so I hope it will help some other sorry arse one day.
Scenario Wanting to render an error page programmatically in my web app (say a fancy 404 page that can’t be served as a static content, as it might contain the current user’s name, etc.)
I catch the error in the app (Monorail, so I use a Rescue, but do whatever feels good), render the markup, and set the ResponseCode to 401.
Problem IIS will render it’s own static 401 ( or any other 40x/50x ) page
I did check that customErrors section is set to off, as it used to work in IIS6 just fine
What I missed out IIS7 introduced the httpErrors section (under system.webServer of course), which is used to tell it what to do with custom errors.
The sub-key I was looking for was <errorMode> with the value set to Detailed.
<system.webserver> ... <httperrors errormode="Detailed" /> ... </system.webserver>
made my problem go away. Also notice that you might need to allow setting that value in your web.config, by going to c:\Windows\System32\inetsrv\config\applicationHost.config, allocating the declaration of the section named “httpErrors” and set its overrideModeDefault attribute to "Allow".