The memcached log has been flooded with error message about preprocess-hash keys since time immemorial. They are of the form:
2015-09-03 00:14:21 mw1083 svwiki memcached ERROR: Memcached error for key "svwiki:preprocess-hash:4a5b991af9ad4714f3175af5da312b1d:1" on server "/var/run/nutcracker/nutcracker.sock:0": SERVER ERROR {"memcached-server":"/var/run/nutcracker/nutcracker.sock:0","memcached-key":"svwiki:preprocess-hash:4a5b991af9ad4714f3175af5da312b1d:1"}The wiki varies, but it's usually zhwiki. The reason for the error is the size of the serialized value, which exceeds memcached's 1 Mb limit.