Goldreich demonstrates the problem by showing that our abstractions of electrical circuits don't in themselves preclude circuits that violate computational complexity results or even decide halting, but that alone is insufficient to show that such circuits can actually be built.

There are certain formalisms that make this kind of error harder to spot: actor formalisms make it easy to hide impossible "computation" in a collaborator; some typed formalisms could hide computation in the syntax (which requires a lot or even infinite computation to decide).