Today, this line made me trouble. Can you spot the mistake?

match 'sitemap.xml' => 'feeds#sitemap', :constraints => { :format => 'xml' }, :as => 'sitemap'

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The mistake is to match sitemap.xml. Rails will by default strip any dot-anything, remember it as desired format and forward the rest of the request to the routing engine. Since we're making .xml part of the match, it is not available for format determination and Rails will set the format to html.

Unfortunately, the constraint won't complain in this case and Rails even renders the sitemap.builder file correctly. My problem only occurred when I wanted to render a customized error page explicitly asking for the format.

So, the correct line would be:

match 'sitemap' => 'feeds#sitemap', :constraints => { :format => 'xml' }, :as => 'sitemap'

This works in Rails 3.