You make it robust by making it simple. Simple to implement, simple to reason about, and simple to debug. They were not only programming an existing computer, they were also more or less defining the computer and this allows some hardware-software co-evolution. In cases like these, the hardware is frozen well before the software is close to ready, so you might need to do some workarounds.

> Handling an expected error condition under normal operation

Nothing in the Apollo program was "normal operation" by any standard. The thing is the alarm elegantly - and correctly - handled a situation that should never have happened in the operation, that only happened because the simulators the procedures were developed against did not accurately account for the workload of having both docking and landing radars on at the same time (IIRC, this was the issue).

This kind of robustness in the face of unexpected misuse is required if you want to qualify your hardware as "man-rated".