'accidentally' set 'CHANGE MASTER TO' on wrong server
In earlier versions of MySQL, you would run this on the Master

STOP SLAVE;
CHANGE MASTER TO master_host = '';


and restart mysqld.

For MySQL 5.5/5.6, you would run this on the Master

STOP SLAVE;
RESET SLAVE ALL;


MySQL restart not required

For more details on this, please read Disconnecting a replication slave is easier with MySQL 5.5+ (RESET SLAVE vs. RESET SLAVE ALL) from PERCONA MYSQL PERFORMANCE BLOG