> every single bit of memory (including registers) would have extra bits for this, and checks would occur everywhere it's moved or used (even over buses inside

> every single bit of memory (including registers) would have extra bits for this, and checks would occur everywhere it's moved or used (even over buses inside IC's, regardless of whether they shift data, addresses, control logic, etc). That's one of the two approaches used for automotive silicon. The other, which is a bit more expensive but usually considered safer, is to run the same circuit twice in parallel (usually with a few cycles of offset) and compare the outputs. If those outputs don't match, the system is reset into a fail-safe mode.