Related to [ZF-7404]. The reason my proposed fix was rather round-about was so that it would also cover the captcha form element. It still doesn't work, however. Before this, it isn't really possible to break apart and render separately a captcha element since important stuff is happening in {{render()}} which is skipped during {{__call()}}. Now it is possible to call {{renderCaptcha()}} and {{renderCaptcha_Word()}}, but they wouldn't ever match because the hidden element's value would be generated twice.

$this->_helper->viewRenderer()->setNoRender();

$form = new Zend_Form();
$form->addElement('captcha', 'captcha', array('captcha' => 'Dumb'))
     ->addElement('submit', 'submit');

if ($this->_request->isPost()) {
    var_dump($form->isValid($this->_request->getPost()));
} else {
    echo $form->renderForm(false);
    echo $form->captcha->renderCaptcha();
    echo '
';
    echo $form->captcha->renderCaptcha_Word();
    echo $form->submit;
    echo '';
}

Action: enter the correct word Expected: true Actual: false

Unit test and proposed solution to follow.