>what if the LLM gets something wrong that the operator (a junior dev perhaps) doesn't even know it's wrong?

the same thing that always happens if a dev gets something wrong without even knowing it's wrong - either code review/QA catches it, or the user does, and a ticket is created

>if it fails here, it will fail with other things, in not such obvious ways.

is infallibility a realistic expectation of a software tool or its operator?

By sheer chance, there's now a HN submission that answers both (but mostly the second) questions PERFECTLY:

https://news.ycombinator.com/item?id=46185957