Adding an Errors decorator to Zend_Form throws an error if any element of the form is not valid and no errors are set for the form.

<?php
require_once 'Zend/Loader.php';
Zend_Loader::registerAutoload();

$oEmail = new Zend_Form_Element_Text('email');
$oEmail
    ->addValidator('EmailAddress', true)
    ->clearDecorators()
    ->addDecorator('ViewHelper')
    ->addDecorator('Errors')
    ->setRequired(true);

$oForm = new Zend_Form();
$oForm
    ->addElement($oEmail)
    ->clearDecorators()
    ->addDecorator('FormElements')
    ->addDecorator('Errors', array('placement' => Zend_Form_Decorator_Abstract::PREPEND))
    ->addDecorator('Form');

$oForm->isValid(array('email' => 'foo'));
$oForm->addErrors(array('Sign up failed')); // Without this throws an error

echo $oForm->render(new Zend_View());