This error occurred in my environment when i tried to create a user in MySQL, most likely you have upgraded MySQL Server (51. to 5.6 in my case) . After upgrading your MySQL you have to run mysql_upgrade to fix any incompatibilities in System tables. If it finds any errors it automatically repairs it and upgrades all the internal data dictionary.
Just run the below command after upgrading your MySQL, restart MySQL and it will fix the issue.
# mysql_upgrade –force -u root -p
# service mysqld restart