how to avoid mysql replication slave change error?

my database structure is

master1 <-----> master2 
   |
   |
slave1

and I want to change my structure to

master1 <-----> master2 
                   |
                   |
                 slave1

my step is 1. master1 : stop slave; and write down the binlog position from matser2 2. wait slave1 to catch up with master1 and stop slave, change master to master2 using step1 binlog position , start slave

and my problem is when I start slave, it occurs

`"Last_SQL_Error: Could not execute Delete_rows event on table hcy.t1; Can't find record in 't1', Error_code: 1032; handler error HA_ERR_KEY_NOT_FOUND; the event's master log mysql-bin.000006, end_log_pos 254"`

can anybody tell me why it occurs and how to prevent ? after skipping slave error and the replication is ok,SQL and IO thread is YES, and after a minutes,the second error occurs

Last_SQL_Error: Could not execute Update_rows event on table hcy.t1; Can't find record in 't1', Error_code: 1032; handler error HA_ERR_KEY_NOT_FOUND; the event's master log mysql-bin.000010, end_log_pos 263

why?