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I have tried the 'sudo rm -rf /' on an old machine that was due to be reinstalled just to see how it would go.

The answer was very far indeed! Eventually the GUI display lost all text when the fonts were deleted, and when the disk LED finally went off I rebooted in to a live CD to view the results - pretty much everything had gone bar a few in-use directories like /proc and similar. rm had even rm'd itself!