If you are setting data to the session and it immediately disappears and you aren't getting any warnings in your PHP error log, it's probably because your sessions expired sometime in the 1970s.
Somewhere between memcached 1.0.2 and 2.1.0, the memcached session handler became sensitive to the 30-day TTL gotcha (aka "transparent failover"). If your session.gc_maxlifetime is greater than 2592000 (30 days), the value is treated as a unix timestamp instead of a relative seconds count.
This issue is likely to impact anyone with long-running sessions who is upgrading from Ubuntu 12.04 to 14.04.
