I was coding along, and I reversed the order of the args in my page to the property_exists() function. After I fixed my error, I searched for any other instances where I'd done that. The editor found this code in libraries/cms/application/administrator.php at line 421:

if (property_exists('JConfig', 'root_user')
    && (JFactory::getUser()->get('username') == $rootUser || JFactory::getUser()->id === (string) $rootUser))
{
    $this->enqueueMessage(
    JText::sprintf(
    'JWARNING_REMOVE_ROOT_USER',
                    'index.php?option=com_config&task=config.removeroot&' . JSession::getFormToken() . '=1'
                ),
                'notice'
            );
        }

I didn't really dig into what it's supposed to be doing, but it's definitely not correct.