Updates of the kind

insert into mysql.tables_priv (host,db,user,table_name,grantor,table_priv) VALUES ('localhost','test','user','t1','root@localhost','Select');

break replication, if it's set up between an older master (having short User field and such) to a newer slave, with increased field length:

Last_Error: Column 2 of table 'mysql.tables_priv' cannot be converted from type 'char(16)' to type 'char(80)'

Such updates shouldn't normally be done, but they are not forbidden and in some cases are useful, e.g. for bulk updates.
Besides, they can come if the data is restored from mysqldump backup/