If something fails in the manner it was assumed it might - and a back-up kicks in, then fine.
But with some automation, you get the scenario where one computer hasn't completely failed, it's just gone a bit icky-poo. So whatever is supposed to take over doesn't, because it still thinks that the first system is OK... See the Virgin Atlantic A340-600 fuel computer AAIB report. In that scenario, what really didn't help was the way the low level fuel indication was overridden by the main system. WTF? Surely it should be completely independent?
I was flying a 4-jet with a split AC system once when a corroded voltage regulator didn't allow the one generator we had left on one side to come off line cleanly when it couldn't cope with all the loads it should following an IFSD on air test. So the system went into 'bugger this' mode and protected the other side. We then had no attitude instruments of any kind whatsoever, no autopilots, half the aileron and elevator PCUs out and one of the rudder PCUs out, a rising cabin altitude and an interesting number of lights and flags in view.....until we restarted the engine and brought its generator back on line, followed by everything else. But being an old fashioned pre-electric jet, the designers had at least thought of a way of coping with such unlikely non-graceful degradation. Or rather the original designers had - we were flying a variant with a considerably modified AC load distribution arrangement and the modification had overlooked the fact that all electrical attitude systems now came off the same side of the AC systems.
Had we dropped the Electrical RAT, we would have recovered some attitude systems. But it was a good job we didn't, because when we tested it a few days later it too had a corroded voltage regulator inside the aircraft which then burnt out on the approach, causing a fair amount of smoke. Which the smoke detector didn't detect, because it was also faulty.
The joys of flying one of HM's museum piece aircraft which had been held 'in storage' in the open at a coastal aerodrome in South Wales.....