After enabling the Cache blocks option @ admin/config/development/performance, the form inside a custom block throws the following error.

• The form has become outdated. Copy any unsaved work in the form below and then reload this page.

One work around is to disable cache for that specific custom block and this could be done by adding the DRUPAL_NO_CACHE in your hook_block_info().

/**
 * Implements hook_block_info().
 */
function <module>_block_info() {
  $blocks = array();

  $blocks['eureka'] = array(
    'info' => t('Eureka'),
    'cache' => DRUPAL_NO_CACHE,
  );
 
  return $blocks;
}

Done =)

Reference: Drupal API – constant DRUPAL_NO_CACHE