One of the SQL Server 2008 upgrade process hampered with the error on the subject line and following text box was displayed:

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For some reason the similar issue hasn't been displayed when Upgrade Advisor has been chosen to find the first hand issues, as per http://sqlserver-qa.net/blogs/tools/archive/2008/01/22/upgrade-blurg.aspx and http://sqlserver-qa.net/blogs/tools/archive/tags/upgrade+advisor/default.aspx posts here.

Coming to the root cause of the issue one of the KBA refers that ....this issue occurs if you renamed the sa account to avoid login attacks on the sa account. Error code 912 indicates that the Sqlagent100_msdb_upgrade.sql upgrade script did not run. I have looked into the upgrade script that is executed which has hard-code reference to use SA account, the error is obvious if you have gone thru rename of SA account procedure on earlier versions of SQL Server, mainly 2005.

As you can see the code on above .SQL file was written to call sp_add_job with an owner_login_name = 'sa'. But if you have renamed 'sa', then sp_add_job will not find the login and attempt to insert a NULL SID into the sysjobs table. The natural reaction should be to just rename 'sa' back and then re-run the upgrade or repair the installation. To resolve this issue, follow these steps (as per BOL):

    1. At a command prompt, run the following command to start SQL Server and to bypass any upgrade scripts:

      net start mssqlserver /f /T3608
    2. To change the account that you renamed back to the sa account, connect to SQL Server by using a dedicated administrator connection (DAC). To do this, run the following query in the sqlcmd command prompt utility:
      sqlcmd -E -A -Q"alter login <sys-admin> with name = sa"
    3. Run the following command to exit SQL Server:
      net stop mssqlserver
    4. Repair the SQL Server 2008 installation. To do this, follow these steps:
      1. Start SQL Server Setup (Setup.exe) from the SQL Server 2008 installation media.

        Note After the prerequisites verification and the system verification, the Setup program displays the SQL Server Installation Center page.
      2. In the navigation pane, click Maintenance, and then click Repair.
    5. To complete the upgrade process, start SQL Server.

Say if the security policies defined to rename SA account, then once the upgrade process is completed and your application is back online functioning with no issues you can simply proceed to rename the sa account again.

As per the error message you must follow through the error log to see any additional errors are occured. Also MSDN blog refers about http://blogs.msdn.com/psssql/archive/2008/09/10/upgrade-for-sql-server-2008-can-fail-if-you-have-renamed-the-sa-account.aspx topic.