When I applied the 10.4.6 Combo updater for Tiger I couldn’t boot up again. The first symptom something was wrong was when I hit the restart button after installation; the thing hung up so I had to do a hard reboot. Afterwards, it would hang once the progress bar of the “Welcome to Macintosh” dialog went all the way across, or in other cases the desktop and cursor would appear and nothing else (no menu bar, dock, etc.).
Running the Applejack suite did nothing to fix the issue (in single-user mode), and I couldn’t boot up in safe mode. Reinstalling the update from another partition didn’t work either.
So while booted in the other partition, I deleted the cache files. The next reboot was fine, and it has been fine since.