On the 'Edit Relationship' page for a release group, the link called 'link' (going to the release group), links to http://musicbrainz.org/release_group/... instead of http://musicbrainz.org/release-group/... (note the underscore vs. hyphen).

Example link:
http://musicbrainz.org/edit/relationship/edit?returnto=http%3A%2F%2Fmusicbrainz.org%2Frelease-group%2F80fe84a4-b606-3491-a170-f2ca17501493%2Frelationships&type1=url&type0=release_group&id=34684