When the link type of a relationship is changed in the mini relationship editor, it is checked if any attributes are valid for the old link type. This may reject flawless edits and accept faulty ones.

(Was mentioned on IRC http://chatlogs.musicbrainz.org/musicbrainz/2014/2014-05/2014-05-22.html#T20-44-02-410516, but no-one bothered to raise a ticket.)