create table t1 as select * from mysql.global_priv;
truncate table mysql.global_priv;
flush privileges;
connect(localhost,u1,,test,MASTER_PORT,MASTER_SOCKET);
connect  fail,localhost,u1;
Got one of the listed errors
insert mysql.global_priv select * from t1;
drop table t1;
flush privileges;
# switching from mysql.global_priv to mysql.user
truncate table mysql.user;
flush privileges;
connect(localhost,u1,,test,MASTER_PORT,MASTER_SOCKET);
connect  fail,localhost,u1;
Got one of the listed errors
# switching back from mysql.user to mysql.global_priv