When using a freshly launched test instance, I tried customising the ui.footer-items configuration variable.

I (accidentally) gave it valid JSON, but not in the expected format (it’s not an array):

{"name": "foo", "href": "example.com"}

Now any page returns

InvalidArgumentException
Argument 1 passed to idx() must be of the type array, string given, called in /core/lib/phabricator/src/view/page/PhabricatorStandardPageView.php on line 719 and defined

The instance is now broken − no page can be displayed.

Hope that helps :)