After dropping a column family, I got an infinite number of messages like this:
WARN [BatchlogTasks:1] 2014-03-07 17:50:08,962 BatchlogManager.java (line 235) Skipped batch replay of bc3916e0-a533-11e3-9c41-55f3810000b5 due to org.apache.cassandra.db.UnknownColumnFamilyException: Couldn't find cfId=2fdad07d-0cc3-373a-8aa0-15837cd9bc6c
I say an infinite number but I only waited about 12 hours before I decided I had had enough and truncated system.batchlog. So perhaps it would have gone away by itself.
Presumably the batch replay is skipped but not discarded, so will keep on turning up forever.