Your Mac OS System Folder may no longer be valid because your System file and other crucial items from the two versions of Mac OS have become mismatched. To fix this, start up from another volume (such as your Software Installer disc) and run the update again to re-synchronize the items.
Sometimes a Mac OS installer system software discs opens an additional installer after the system software installation is complete. This extra installer may replace certain system files such as the Mac OS ROM, System Resources and System file with versions intended for use with its (older version) of Mac OS. Installing any older Mac OS item from its installer still causes this extra installer to open. Unfortunately, this additional installer does not check to see if a newer item is installed and instead replaces the items that were updated in the newer version of Mac OS already installed on the hard disk. Although these items are inadvertently replaced with older versions, the version of the Finder and other items installed are not replaced, so you end up with a non-functioning System Folder.
If you have a Mac OS full install or update disc, you can start up from it and simply install the update to repair the System Folder. If you downloaded the update and it is still on your hard disk, you can start up from the older Mac OS disc and run the updater from the hard disk to repair the System Folder, too.
Firmware
The discussion above applies to firmware updates, too. If a firmware update has been installed, using an older Mac OS system software installer may cause the same difficulties.