When MySQL attempts to start up again, it won’t be able to find the relay log and replication will fail. The error messages in /var/log/mysqld.log might look something like:

[ERROR] Failed to open the relay log '/var/run/mysqld/[relay log filename]'

The pre-emptive fix is to tell MySQL to store the logs elsewhere (/var/lib/mysql/ perhaps) using the relay-log configuration option. If you’ve already suffered this problem, then the most straight forward fix is to start again; do a show master status, flush logs and then a mysqldump to get a fresh starting point on the master, and then copy the new dump over to the slave.

More information on the mysql bug report.