Software is not always like hardware though: the same software failure can cascade across redundant instances rapidly just as easily as it affects the primary.

The same hardware failure can cascade too; redundancy alone is not a panacea, you need some safeguards. Ideally, nothing should fully trust anything else.

I was just reading that they design the rocket so that one engine could explode without affecting the others (hopefully) and they test the engines by dropping a nut or bolt or something in, which is not something rocket engines normally can tolerate.

Maybe cascade was the wrong word. I meant that if you pop in a hot spare and it's running the same software, there's a good chance it will encounter the same issue, sometimes immediately.

In some cases redundant software is developed independently by different teams to the same spec; I think I read about that with the Space Shuttle.

How correlated are your layers of swiss cheese...?