Fatal error: Call to undefined function colorbox_gallery_custom_validate() in /code/includes/form.inc on line 1459

Apparently

  $element['colorbox_settings']['colorbox_gallery_custom'] = array(
    '#title' => t('Custom gallery'),
    '#type' => 'textfield',
    '#maxlength' => 32,
    '#default_value' => $settings['colorbox_settings']['colorbox_gallery_custom'],
    '#description' => t('All images on a page with the same gallery value (rel attribute) will be grouped together. It must only contain lowercase letters, numbers, hyphen and underscores.'),
    '#element_validate' => array('colorbox_gallery_custom_validate'),
    '#required' => FALSE,
    '#states' => array(
      'visible' => array(
        ':input[name$="[settings][colorbox_settings][colorbox_gallery]"].picture-colorbox-gallery' => array('value' => 'custom'),
      ),
    ),
  );

colorbox is required :)

It would be nice to wrap this in a module_exists() or introduce a dependency.