PHP Fatal error: Uncaught exception 'Mage_Core_Exception' with message 'Mage registry key "_singleton/core/session" already exists'
You’ll see this if you try and do
from a command-line magento script which hasn’t had the session initialised already. A simple workaround is to load the session model yourself early in your script:
$session = Mage::getModel('core/session');