I've had this problem for several hours, and now I'm just tired..
When I try to start MySql from terminal (mac), i get
ERROR! The server quit without updating PID file (/usr/local/mysql/data/127.0.0.1.pid). There seem to be something wrong with the table mysql.plugin, log file says it doesn't exist.
I have uninstalled and reinstalled mysql on several ways, trying every method i have found googling around. Therefor i think there must be something wrong with the computer - not the mysql software.
Latest install is from mysql.com, a dmg-file.