This has happend to me a few times. You import a mysql user database from another server and you get these horrible errors when you restart mysql.
error: 'Access denied for user 'debian-sys-maint'@'localhost' (using password: YES)'
Go find your debian-sys-maint password in
Write it down you will need it later. Log on to your mysql server and issue the following command
GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'your-password' WITH GRANT OPTION;
Change your-password with the password from the file debian.cnf. I am sure the debian-sys-maint user could be given less rights but this thread is only helping to restore the previous defaults. So please be aware this could be a security issue.