Normal user dropping system user

User without `system_user` privilege is able to drop system users.

Replication issue:

1) Log in as root and create 1 normal user & 1 system user. 

Create user x (normal user) with create user privilege.

Create user y (system user) with all privileges.

2) Log in as user x and drop user y.

Error with access denied. But following selects will have error with WSREP not prepared. (due to inconsistency with other nodes)

3) Check mysql error log:

origin node initiated vote with access denied operation, other nodes vote as succeed hence user y will be dropped. origin node will proceed to IST from other pxc nodes.

4) When origin node is ready, user y has been dropped. 

Commands and logs are written in percona forum