public function testAction()
    {
       $subForm1 = new Zend_Form_Subform(array('legend'=>'Subform 1'));
       $subForm2 = new Zend_Form_Subform(array('legend'=>'Subform 2'));
       $subForm3 = new Zend_Form_Subform(array('legend'=>'Subform 3'));
       
       $subForm3->addElement('checkbox', '1', array('label' => 'Checkbox'));
       $subForm3->addElement('text', '2', array('label' => 'Textbox'));
       
       $subForm2->addElement('checkbox', '1', array('label' => 'Checkbox'));
       $subForm2->addElement('text', '2', array('label' => 'Textbox'));
       
       $subForm2->addSubform($subForm3, '3');
       $subForm1->addSubForm($subForm2, '2');
       
       $form = new Zend_Form();
       $form->addSubform($subForm1, '1');
       
       $form->addElement('submit', 'submit', array('label' => 'Submit'));
       
       if ($this->getRequest()->isPost() && $form->isValid($this->getRequest()->getPost())) {
            echo "Succeeded";
       }
       
       $this->view->form = $form;
    }

This throws the following exception:

exception 'Zend_Form_Exception' with message 'Zend_Form::Zend_Form::isValid expects an array' in /Users/Jason/Sites/asdf/library/Zend/Form.php:1973 Stack trace: 
#0 /Users/Jason/Sites/asdf/library/Zend/Form.php(1995): Zend_Form->isValid('') 
#1 /Users/Jason/Sites/asdf/library/Zend/Form.php(1993): Zend_Form->isValid(Array) 
#2 /Users/Jason/Sites/asdf/library/Zend/Form.php(1993): Zend_Form->isValid(Array) 
#3 /Users/Jason/Sites/asdf/application/modules/default/controllers/TestController.php(47): Zend_Form->isValid(Array)

Things to note:

1: This will work fine when the elements from subform 2 are removed 2: This will work fine when subform 3 is removed entirely 3: When all element names are unique and not the same name of any subform it works fine, but if you name the elements '1', '2', '3', and '4' respectively, subform 3 won't be displayed, and the form will still fail.