Sometimes I hear idea about storing user session data in memcache. It is great idea, memcache is fast, easy to scale, cached data can be shared between many servers, so it is not necessary to take care about advanced load balancing. Yeach it is a trap. Even official Memcached FAQ says that:
http://code.google.com/p/memcached/wiki/NewProgrammingFAQ#Why_is_memcached_not_recommended_for_sessions?_Everyone_does_it!
http://dormando.livejournal.com/495593.html
To prove that described situation can easily happened I wrote some example code available here https://mrok@github.com/mrok/memcache_test.git. Download it, fix memcache configuration section, run and wait for first missing key. Do you still think it is good idea?