You can see this error when restarting MySQL. The system has detected that MySQL is running when it is not. This is probably due to and hard reboot of the system.
To fix this problem, remove the file
/var/lib/mysql/mysql.sock as root user:
# rm /var/lib/mysql/mysql.sock
Then start the MySQL daemon:
# service mysqld start
# /etc/init.d/mysqld start