I installed the patch file PATCH_SUPEE-6788_CE_1.9.2.1_v1-2015-10-26-11-38-41.sh on a Magento 1.9.2.1 site and the site would fail to load with these errors:
Class 'Zend_Xml_Security' not found in /var/www/html/magento/lib/Zend/Locale/Data.php on line 160
Class 'Mage_Core_Helper_UnserializeArray' not found in /var/www/html/magento/app/code/core/Mage/Core/Helper/UnserializeArray.php on line 43
Class 'Unserialize_Parser' not found in /var/www/html/magento/app/code/core/Mage/Core/Helper/UnserializeArray.php on line 43
I found that the patch creates each of these new classes in new files, and the files were created properly with the right ownership and permissions, but these new classes were not being loaded properly by Magento at runtime. I tried manually adding UnserializeArray to the var/package/Mage_Core_Modules-1.9.2.1.xml file, which eliminated that particular error but not the other fatal errors above.
Why is the patch not written to provide for properly loading these new classes? It appears the patch is completely defective. Is there a fix for this, or will I have to choose between an unpatched site or an upgrade to a new version?