Comment #5 on issue 243 by [email protected]: Internal error! Invalid slab class
http://code.google.com/p/memcached/issues/detail?id=243
Can you try http://memcached.googlecode.com/files/memcached-1.4.11_beta1.tar.gz and see if it fixes your error? (or -rc1 or -final whenever you read this).
