1. Uninstall tool_task
2. Visit site administration page
3. It's not happy:

   Exception - Class "tool_task\check\cronrunning" not found

   More information about this error

   Debug info:

   Error code: generalexceptionmessage

   Stack trace:

   line 610 of /admin/renderer.php: Error thrown

   line 306 of /admin/renderer.php: call to core_admin_renderer->cron_overdue_warning()

   line 938 of /admin/index.php: call to core_admin_renderer->admin_notifications_page()

Regression from MDL-47271, although some of it pre-dates that from MDL-64454 when reading plugin config from admin page

There is another problem from the same page, in that the plugin config is defined in a core settings file, causing the following because neither the plugin or its lang strings exist:

String does not exist. Please check your string definition for enablerunnow/tool_task

    line 164 of /lib/classes/lang_string.php: call to debugging()

    line 88 of /admin/settings/security.php: call to core\lang_string->__construct()

    line 8855 of /lib/adminlib.php: call to require()

    line 835 of /admin/index.php: call to admin_get_root()

This breaks our rules around core/component communication, which exist specifically to prevent this

From related discussion with maherne in dev chat