Dual-core lockstep
Two identical CPU cores run exactly the same program, and a comparator raises the alarm the moment they disagree.
The outputs of both cores are compared every clock cycle. Often one runs a couple of cycles behind the other, so that a glitch on the supply rail doesn't hit both in the same state. On a mismatch, a fault is signalled and the system drops into a safe state. The scheme detects errors but can't tell which core is right — with two votes there is no majority.
Lockstep is standard in automotive and industrial microcontrollers, where ISO 26262 demands that random hardware faults be caught. Arm's Cortex-R5 can be configured this way, and Infineon's AURIX carries checker cores for the same purpose. Half the silicon does no useful work at all; it just stands there watching the other half.