For one relationship I got a message "link type id is required" or something like that in red and centre-aligned. That sounds a bit technical. I would expect something like "The relationship type is required." and not centre-aligned.
When I wanted to reproduce that by selecting an artist and a release and submitting the relationship without selecting a type, I got an internal server error.
The URL for that was: http://musicbrainz.org/edit/relationship/create?type1=release&entity1=d6221af8-5aa8-4ebe-88d2-c4e42fbf2ebc&entity0=fa96eaf2-ac07-415e-a060-059fb4bbe8b2&type0=artist