The problem with that is that most programming languages aren't sufficiently expressive to be able to recognise that, say, only a subset of switch cases are act

The problem with that is that most programming languages aren't sufficiently expressive to be able to recognise that, say, only a subset of switch cases are actually valid, the others having been already ruled out. It's sometimes possible to re-architect to avoid many of this kind of issue, but not always. What you're often led to is "if this happens, there's a bug in the code elsewhere" code. It's really hard to know what to do in that situation, other than terminate whatever unit of work you were trying to complete: the only thing you know for sure is that the software doesn't accurately model reality. In this story, there obviously was a bug in the code. And the broken algorithm shouldn't have passed review. But even so, the safety critical aspect of the complete system wasn't compromised, and that part worked as specified -- I suspect the system behaviour under error conditions was mandated, and I dread to think what might have happened if the developers (the company, not individuals) were allowed to actually assume errors wouldn't happen and let the system continue unchecked.