// Hide q8 if q9 checkbox is checked.
$form['some_question[q8]'] = array(
  '#states' => array(
    'invisible' => array(
      ':input[name="some_question[q9]"]' => array('checked' => TRUE),
    ),
  ),
);
// Hide q9 if q8 checkbox is checked.
$form['some_question[q9]'] = array(
  '#states' => array(
    'invisible' => array(
      ':input[name="some_question[q8]"]' => array('checked' => TRUE),
    ),
  ),
);

// Hide q8 if q9 checkbox is checked.
$form[name="some_question[q8]"] = array(
  '#states' => array(
    'invisible' => array(
      ':input[name="some_question[q9]"]' => array('checked' => TRUE),
    ),
  ),
);
// Hide q9 if q8 checkbox is checked.
$form[name="some_question[q9]"] = array(
  '#states' => array(
    'invisible' => array(
      ':input[name="some_question[q8]"]' => array('checked' => TRUE),
    ),
  ),
);

DrupalComponentSerializationExceptionInvalidDataTypeException: Unable to parse at line 1 (near "':input[name="some_question[q8]"]' = array("). in DrupalComponentSerializationYamlSymfony::decode() (line 40 of  .. wwwd8sitewebcorelibDrupalComponentSerializationYamlSymfony.php).