This was a problem in mason - http://bugs.musicbrainz.org/ticket/5338

I don't know if it's still happening to new edits or if these URLs are just left over from then, but we do have some unused URLs which haven't been removed, e.g. http://musicbrainz.org/url/5bbacea4-6141-45c9-ab92-40992de6c747/relationships and http://musicbrainz.org/url/2db97993-3696-4cf4-bec2-9b2d0d8858c2/relationships