Broken folder icon: what exactly causes it?

We have previously noted a startup symptom where you get a broken (or cracked) folder icon at Mac OS X startup. As we have never personally had this symptom (not that we're complaining!), it has been difficult for us to diagnose exactly what causes it.

A posting in a MacFixIt Forums thread (Cracked startup icon) claims that the cracked folder icon indicates a problem with Open Firmware - and that updating to and using the latest 9.2.1 version of the Startup disk control panel will likely fix it.

Our previous posting has linked it to a third-party SCSI drives.

Update: Rick Barnett adds: "I encountered the broken folder icon when I attempted to boot from an external FireWire drive upon which I had installed Mac OS X. The bootup stopped short of the desktop. It reverted to a gray screen with the broken folder icon. The subsequent gray screen included arrow selections which brought up bootable volume choices from which I was able to successfully boot the system."