The guix-download test includes this snippet:

------
# Make sure it fails here.
if guix download http://does.not/exist
then false; else true; fi
------

Unfortunately, many ISPs (such as T-Mobile) return bogus results for
otherwise unclaimed domain names, causing this test to fail.

Does anyone know if there is some domain that is designed to fail as a
"standard", as <http://example.com> is intended to be used for examples
of good domains?