1/ install Moodle 2.1
2/ unpack Moodle 2.3 over it
3/ observe the new error message
Admins often forget to remove old PHP files when doing major upgrades - http://docs.moodle.org/22/en/Upgrading
Solution is add a new upgrade test for files that should not be there any more...
Plugins missing from disk and cant delete them
Check for removed files before CLI upgrade