Get an application to hang in Mac OS X. Force a system shutdown without quiting the application. If you are unlucky you will have corrupted /sbin/mach_init and /mach.sym. Disk Utility and Disk Warrior will report both files as overlapping files. Disk Utility will lie to you outright and tell you they are fixed. Disk Warrior will at least report that they are unfixable, or at least obviously fail to fix them. Single user mode will halt at a /sbin/mach_init, errno 2 prompt. Replacing the files via target firewire disk mode and repairing permissions should be enough to remedy the situation, but it won’t be. And an Archive and Install will refuse to work on a drive without an intact mach.sym. Searching the support boards and talking to multiple tech support personnel will leave you with dead ends and solutions you don’t want to read or hear.
After you’ve done all this back up the user data, prep your Erase and Install disks, and don’t forget to give Apple a piece of your mind.