Make sure incoming vids are free tagging enabled.

File

drupal-6.x/modules/taxonomy/taxonomy.module, line 652

Enables the organization of content into categories.

Code

function taxonomy_node_validate(&$node) {
  if (!empty($node->taxonomy)) {
    $terms = $node->taxonomy;
    if (!empty($terms['tags'])) {
      foreach ($terms['tags'] as $vid => $vid_value) {
        $vocabulary = taxonomy_vocabulary_load($vid);
        if (empty($vocabulary->tags)) {
          // see form_get_error $key = implode('][', $element['#parents']);
          // on why this is the key
          form_set_error("taxonomy][tags][$vid", t('The %name vocabulary can not be modified in this way.', array('%name' => $vocabulary->name)));
        }
      }
    }
  }
}