I recall working on some Sun machines with hot-swappable CPUs (and, I assume, disks and other peripherals). If they somehow made memory hot swappable (I'm sure it's possible, just uncommon and/or verrry expensive), with hot swap CPUs and disks, and redundant power supplies, you could tear the machine half apart and it would still keep running. Of course, at that point, once everything is hot swappable, there are generally multiples of everything, so your one machine is really more like multiple machines inside one box than a single discrete machine.