At least in airliners, the truly critical computers are redundant. Typically three identical copies of the autopilot computers run in parallel and compare results; if one computer disagrees with the other two, its output is ignored. The system allows some processors to be faulty while maintaining the operation of the overall system.
But why? I've never heard of microprocessors suddenly failing. Sure, there could be manufacturing errors, but those would have been caught at the factory. Perhaps the program (and its proof) is wrong, but it would be wrong in the same way across the processors. Similarly, bad input would cause bad output across all three computers. What kind of errors does this redundancy protect against? Do microprocessors sometimes just do math wrong?
If a microprocessor is overheated or overloaded and spontaneously fails, I would expect it to stop doing anything and produce no output. To deal with this kind of failure, you'd want to have a backup processor, but you wouldn't need to compare the outputs of three computers—any output produced would be assumed correct, so you'd be happy to directly use the output of any processor that was producing output.
Related: The answer to What is the purpose of multiple autopilots? simply says "redundancy" before going into how this is achieved.