But what do you do when there is a bug with the hardware clock, or with the image analysis software that recognizes how far the ground is? Or, supposing you installed a laser measurement tool to gauge distance-to-ground level, with the software interpreting that? You could of course have quorums - with the majority of the systems deciding which solution (still a while to go / imminent crash!) is the correct one, but what if you had a problem with that part? And that's just the problem of figuring out that there is a problem.

>The first thing to do was to try to seal up the hole. This turned out to be impossible, because the ship’s sensors couldn’t see that there was a hole, and the supervisors, which should have said that the sensors weren’t working properly, weren’t working properly and kept saying that the sensors were fine. The ship could only deduce the existence of the hole from the fact that the robots had clearly fallen out of it, taking its spare brain-which would have enabled it to see the hole-with them.

You could stack layer upon layer of fault detecting modules and supervising software and error and redundancy modules, but then you'd probably wind up with something like Dark Star's Bomb 20:

>Unfortunately, Doolittle has mistakenly taught the bomb Cartesian doubt and, as a result, Bomb #20 determines that it can only trust itself and not external input. Convinced that only it exists, and that its sole purpose in life is to explode, Bomb #20 states "Let there be light," and promptly detonates.