(Don’t) read data from memcache to write back to a database.
(Don’t) forget the negative caching case.
(Don’t) set the cache timeouts too low.
(Don’t) forget to lock.
(Don’t) store low read/write ratio data.
(Don’t) cache data too early.
(Don’t) store large things
(Don’t) store images
(Don’t) rely on memcache as a persistence layer.