Why are critical flight computers redundant?
Failure modes to consider:


Overheating. This changes the chip's timing properties and eventually results in error. This can manifest as single-bit errors in the middle of seemingly normal operation; it will eventually crash, but may output bad data first.
Water damage. Manifests as a parasitic resistance on the board and may cause you to misinterpret bits as high or low. May be leaky housings, condensation, etc.
Electromagnetic interference. (The system is supposed to be resistant to this, but it's still worth thinking about).
Physical connection problems. Either during construction (soldering faults) or induced afterwards (heat, vibration). Microscopic cracks in boards or joints can pass QA but result in intermittent faults. Again this can lose you a single bit at a time. This is related to the Xbox "red ring of death" issue.
Failure of other components. Capacitors are the usual suspect; electrolytic, tantalum, ceramic all have different failure modes. Again this may result in a system that mostly works but is prone to misinterpreting marginal values or suffers from timing drift.
Weird materials science ("Purple plague", tin whiskers due to lead-free solder)
Part QA may not be up to the standards you expect (suppliers shipping inferior parts with a bogus "aerospace grade" label). Hard to detect even after it's happened.


It's important to recognize that in high-speed digital systems you don't get nice clean "one" and "zero", you get a series of rising and falling edges that are smeared out by the parasitic capacitance and inductance of the wiring. This is inherently vulnerable to being misinterpreted in marginal electrical conditions.