Also make shure app/code/core/Mage/Core/Model/Session.php are not rewrited (check local/Mage/Core/Model/Session.php)

Also make sure you do not modify app/code/core/Mage/Core/Model/Session.php directly, but copy the file to app/code/local/Mage/Core/Model/Session.php and make the appropriate modification.

(Magento will check the local directory path before using the core path.