Replication on MySQL server
If your error is  

ERROR 1129 (HY000): Host 'serverb' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts' 

you can run a FLUSH HOSTS on master, it will flush the list of blocked host which exceed the number or fail connections attempts to your mysql master

It won't affect any running connection, it will only clean the list of blocked hosts.