Certain errors (unhandled exceptions, I guess) don't result in the usual WMF error page. See the description of T110878:
What is interesting is that on WMF sites, this doesn't show the usual "Wikimedia Foundation error" page, but instead shows a quasi-plain text response of a kind that I have never seen before: https://www.mediawiki.org/wiki/Special:WhatLinksHere/Media:Wiki.png The HTML source of the returned page is exactly the following:MediaWiki internal error.<br /> <br /> Exception caught inside exception handler.<br /> <br /> Set $wgShowExceptionDetails = true; at the bottom of LocalSettings.php to show detailed debugging information.<br />
This error is unhelpful to non-developers, because it uses highly technical jargon like "exception caught inside exception handler", and doesn't tell users where to report the error. It doesn't even show the skin interface, which may make some users worry that they have "broken the site"!