Upgrade Memcached Before WordPress
Self-hosted WordPress and WordPress MU administrators: if you are using the memcached object cache (a prerequisite for batcache), upgrade it before upgrading WordPress. There is a bug that keeps the old db_version in the options cache, preventing WordPress from remembering that it has been upgraded, and this causes it to ask you to upgrade again. In a pinch you can resolve the problem by restarting the memcached daemon.
The memcached object cache can’t be automatically upgraded because it’s not a normal plugin. Make sure to use the right version: 1.0 (sockets) or 2.0 (PECL). Only one line was changed, so you might prefer to update by hand: 1.0, 2.0.
If you aren’t sure whether you are using memcached, look for a file named object-cache.php in wp-content. If that file exists, look inside to see if the plugin name is “Memcached”.