comment form custom validation
The values for comment_body is not stored in $form_state but in something like:

$form_state['values']['comment_body']['und'][0]['value']

function banana($form, &amp;$form_state) {
  if ($form_state['values']['comment_body']['und'][0]['value'] == "example") {
    form_set_error('comment_body', 'Your comment can not contain example!');
  }
}