My mysqld runs on Ubuntu Hardy and after replacing the complete mysql data with a dump from another server the "/etc/init.d/mysql stop" command fails, because the system administration account doesn't work anymore. So I had to recreate it again:
GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'password' WITH GRANT OPTION;
The password can be obtained from /etc/mysql/debian.cnf.