[Plugin: WordPress Automatic upgrade] Upgrade failed

I tried upgrading 2.3.1 to 2.6 using this automatic tool. At first, I skipped steps 1 and 2 because I tried to upgrade my backup version of WP. Then it failed on downloading latest version (because the folder wpau-backup in the root of my site was non-existant.
Then I created that folder and tried again- this time the update was successful, but the site didn’t work after upgrade. Instead, I just saw a bunch of PHP notice’s about the use of undefined constant- COOKIEHASH.
I started the upgrade from scratch, but this time I set WP_DEBUG to false in my wp-config.php. After this, everything succeeded. Well, my themes need to be tweaked but everything works so far.

http://wordpress.org/extend/plugins/wordpress-automatic-upgrade/