I had a small problem with upgrading the CCK module during the transition from Drupal 5 to Drupal 6. I use the Link module to have a link field (the field name is "url") in a custom content type (named "link"). However after the upgrade whenever I added or updated a node of this content type, the value of the url field was empty. In the Drupal log I saw errors like this:
Column 'field_url_title' cannot be null query: INSERT INTO content_type_link (vid, nid, field_url_url, field_url_title, field_url_attributes, field_link_value) VALUES (4440, 4439, 'http://example.com/', NULL, 'N;', '<a href=\"http://example.com//\">Some link</a>') in /var/www/mysite/sites/all/modules/cck/content.module on line 1200.