Get following code:

$form = new Zend_Form();
$form->addElement(new Zend_Form_Element_Submit('a',array('label'=>'a')));
$form->addElement(new Zend_Form_Element_Submit('b',array('label'=>'b', 'order'=>0)));
$form->addElement(new Zend_Form_Element_Submit('c',array('label'=>'c', 'order'=>1)));
echo $form;

Only submits 'b' and 'c' are displayed.

Reason: In Zend/Form.php (@version $Id: Form.php 8996 2008-03-22 16:59:06Z matthew $) there is:

2507:  if (array_search($index, $this->_order, true)) {
2508:      ++$index;
2509:  }
2510:  $items[$index] = $key;

In example above, this condition is true for index equal to 0 AND 1, but because there is just if, it is executed only once, so index is set to 1. To $items[1] is set submit 'a'. But it is overwritten by submit 'c', because submit 'c' has order set explicitly so no check is performed.

Solution:

2507:  while (array_search($index, $this->_order, true)) {