Why would Craft produce a template loader error (TemplateLoaderException) when rendering its default error pages?
As it turns out, I had made a configuration change that broke Craft's default behavior:

'defaultTemplateExtensions' =&gt; array('twig', 'xml')


All of Craft's internal templates use .html, and HTML templates are included in the default value of that config item. By overriding it, I had rendered Craft unable to find its stock error template.

Moral of the story &mdash; When you override defaultTemplateExtensions, make sure to include the default file extensions ('html','twig') in the new list that you set.