Some relationships have required attributes, such as the performed instrument relationship - http://musicbrainz.org/edit/relationships/edit_link_type.html?type=artist-track&id=59054b12-01ac-43ee-a618-285fd397e461 says the minimum number of instruments is 1 and if you try submitting http://musicbrainz.org/edit/relationship/add.html?link0=track=5367417&link1=artist=36434&linktypeid=3 without selecting an instrument it'll complain.

In NGS, selecting the performed instrument relationship on http://test.musicbrainz.org/edit/relationship/create?type1=recording&entity1=58b05e0e-17a5-4fea-85b6-65d853566f8e&entity0=7e84f845-ac16-41fe-9ff8-df12eb32af55&type0=artist and submitting the form will enter the relationship even though it shouldn't - http://test.musicbrainz.org/recording/58b05e0e-17a5-4fea-85b6-65d853566f8e/relationships