I have a custom form and i render the form using drupal_get_form() inside the .tpl.php. One day, i find that when the submission could not pass the validation, only the failed field was highlighted but without the error message. Then i find the following post suggesting that i should called the drupal_get_form() in the .module file or preprocess function instead of the .tpl.php.
form_set_error Not displaying error after invalid form submission

So i put the drupal_get_form() call in the preprocess function and set the form in the $vars so it could be printed in the .tpl.php. But that still could not solve the problem. Finally i come up with a simple solution which is just adding the following line in the .tpl.php.

<div><?php print theme_status_messages(array('display' => 'error')); ?></div>

Done =)

Reference:

• form_set_error Not displaying error after invalid form submission
• StackOverflow – form_set_error not working
• Drupal API – theme_status_messages