Mysql's internal mysql database in replication

I searched around but the word game seems to delude any valid result and could not find anything relevant.

Mysql (the RDBMS) has an internal database named mysql, this database has information about the users and as far as I know, information and settings about the server running the RDBMS.

Background Info

In our set up we have a few MySQL 5.5 servers and one test MySQL 5.6 server.

Replication is set up so that the 5.6 is a slave of one of the 5.5

Incident

This week the replication to the 5.6 stopped working, when doing a show slave status this error was reported:

Error: Error Column count doesnt match value count at row 1 on query. 
Default database: mysql. 
Query: INSERT INTO tmp_user VALUES (localhost,root,,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,Y,,,,,0,0,0,0,,)

When I checked, the mysql database did not have a tmp_user table, however the error is that the column count doesnt match, what I expected was for the error to be that the table did not exist (Error 1146).

Another conclusion that I gathered from this situation is that we are replicating the database mysql.

Questions

1. Is there any concern/benefit in replicating the mysql database?
2. Do you see this scenario as an injection attack? or a security concern? (Perhaps I am missing a scenario where creating/populating/deleting a tmp_user table is normal)

Thanks for your time