I upgraded my php on a site with memcached plugin already installed and configured for use so that memcached was then NOT installed.
This caused a fatal error:
Fatal error: Class 'Memcached' not found in /Users/danp/git/integration/cache/stores/memcached/lib.php on line 124'
I suppose you could reproduce it by:
1/ Enabling a memcached plugin and setting it as a store
2/ Disabling the memcached pph exntension in your php.ini
3/ Visiting the site