On any wiki without a matching chapter wiki, [[chapter:foo]] creates invalid links.
e.g. on it.wp, [[chapter:foo]] creates a valid link to subdomain https://it.wikimedia.org (which redirects to a different site)
however on en.wp, it creates an invalid link to subdomain https://en.wikimedia.org/wiki/foo , which is server not found!
https://en.wikipedia.org/w/api.php?action=query&meta=siteinfo&siprop=interwikimap and https://meta.wikimedia.org/wiki/Special:Interwiki has an entry:
chapter //en.wikimedia.org/wiki/$1 yes
Recently removed and re-added on 1 March 2013 by
https://gerrit.wikimedia.org/r/#/c/51365/
https://gerrit.wikimedia.org/r/#/c/51367/