On the front-office side, the SiteCache class asks the back-office for "_sites.xml", and parses the result.
If the back-office's configuration is invalid, the response is the "CMS configuration invalid" page instead of XML, which can't be parsed and makes the front-office application crash.
In that case, the back-office should send back a server error, and the front-office handle it and display a proper "unavailable site" error page.