I have a server running Debian 5.0, and MySQL. Suddendly, MySQL stopped working, and after many attempts to fix it, I decided to reinstall it. I installed MySQL 5.1.63, and when started it goes to safe mode. I made some typing, and when I executed mysql_upgrade as root, it complained:

...
Running 'mysql_fix_privilege_tables'...
ERROR 1548 (HY000) at line 1111: Cannot load from mysql.proc. The table is probably corrupted
ERROR 1064 (42000) at line 1112: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'sqlstate 'HY000' set message_text='Unexpected content found in the performance_s' at line 1
ERROR 1548 (HY000) at line 1125: Cannot load from mysql.proc. The table is probably corrupted
FATAL ERROR: Upgrade failed

I checked the mysql.proc table, and it's comment column was slightly different from my backup.

-- My backup says:
`comment` char(64) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT '',
-- But it were:
`comment` text CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,

So, I restored my mysql database backup, and now they all match, but mysql_upgrade still trigger the same errors. I also tried do check and repair the mysql.proc table, but got no success.