Logging in on test2.wikipedia.org should respond to the POST with a redirect back to the page I was reading, but instead serves an entirely empty page. Not even an "Internal error" error page from MediaWiki, or a generic 500 handler from Varnish either. Just nothing.
[WZOHDApAMFAAAFYiPVwAAAAO] /w/index.php?title=Special:UserLogin&returnto=Special:Notifications ErrorException from line 107 of /srv/mediawiki/php-1.30.0-wmf.14/extensions/AntiSpoof/SpoofUser.php: PHP Error: Argument 1 passed to SpoofUser::batchRecord() must be an instance of Wikimedia\Rdbms\Database, Wikimedia\Rdbms\DBConnRef given
That fact that this kind of exception can result in no error page to the user is its own problem.