Invalid markup

in template.php
paste

function THEME_tablefield_view($variables) {
  $attributes = array(
    'class' => array(
      'tablefield'
    ),
  );

  return '<div>'
    . theme('table',
      array(
        'header' => $variables['header'],
        'rows' => $variables['rows'],
        'attributes' => $attributes,
      )
    )
    . '</div>';
}

clear cache

This is a very old issue, and I cannot reproduce it today, as each table now has a unique ID. If the problem persists please reopen.