Can't find file: './mysql/plugin.frm' (errno: 13 - Permission denied)
The reason this happens seems to be that some upgrade process either in brew or in os x changes the permissions for mysql. The solution is simple:
sudo chown -R _mysql /usr/local/var/mysql sudo chmod -R o+rwx /usr/local/var/mysql
This solves the problem for me on Mavericks using mysql installed from brew.