Initialise a behat site Navigate to the site page. Do not log in Edit index.php in your preferred editor Just before the line redirect_if_major_upgrade_required (around line 35 add: throw new \coding_exception('Oops'); Refresh your browser Confirm you get the Whoops error page showing you the full trace and a code snippet Grab the behat_wwwroot from your config.php Navigate to your Behat wwwroot Confirm that you saw an old school error page and not the Whoops one