Relay log read failure: Could not parse relay log event entry

It happened today... why? Well, I was testing parallel replication (slave_parallel_threads) and after setting the number of threads to zero and when started the slave I got this wonderful error message:

MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: master.example.com
                  Master_User: user
                  Master_Port: 3306 
                Connect_Retry: 60
              Master_Log_File: bin.077287
          Read_Master_Log_Pos: 12307299
               Relay_Log_File: relay.167660
                Relay_Log_Pos: 52428684
        Relay_Master_Log_File: bin.076470
             Slave_IO_Running: Yes
            Slave_SQL_Running: No
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 1594 
                   Last_Error: Relay log read failure: Could not parse relay log event entry. The possible reasons are: the master's binary log is corrupted (you can check this by running 'mysqlbinlog' on the binary log), the slave's relay log is corrupted (you can check this by running 'mysqlbinlog' on the relay log), a network problem, or a bug in the master's or slave's MySQL code. If you want to check the master's binary log or slave's relay log, you will be able to know their names by issuing 'SHOW SLAVE STATUS' on this slave.
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 49487635
              Relay_Log_Space: 42906299762
              Until_Condition: None 
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: NULL 
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 1594 
               Last_SQL_Error: Relay log read failure: Could not parse relay log event entry. The possible reasons are: the master's binary log is corrupted (you can check this by running 'mysqlbinlog' on the binary log), the slave's relay log is corrupted (you can check this by running 'mysqlbinlog' on the relay log), a network problem, or a bug in the master's or slave's MySQL code. If you want to check the master's binary log or slave's relay log, you will be able to know their names by issuing 'SHOW SLAVE STATUS' on this slave.
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 1220 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
                   Using_Gtid: No
                  Gtid_IO_Pos: 
1 row in set (0.00 sec)

To recover from this situation, I had to instruct MySQL to clear out all the relay-bin-logs beyond the following point:

Relay_Master_Log_File: bin.076470
Exec_Master_Log_Pos: 49487635

by doing the following:

STOP SLAVE;
CHANGE MASTER TO MASTER_HOST='master.example.com', MASTER_USER='user', MASTER_PASSWORD='password', MASTER_LOG_FILE='bin.076470', MASTER_LOG_POS=49487635;
START SLAVE;

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