[WhipGhostrider]

I have recently worked on software testing for a roughly analagous project to Starliner. The answer is thorough simulations and ground testing. Flight code should have essentially 100% coverage in unit tests as well.

You can do a pretty decent simulation of a space mission on a powerful cluster. You then plug in your simulated spacecraft and run the real flight software. Reentry is easily the most critical phase of flight for a crewed spacecraft, and they should have absolutely tested and caught this flaw in simulation. Ground testing could have also checked on the actual spacecraft that the correct valves were actuated when commanded to.

If I were to speculate, this suggests a pretty large disconnect between the software and hardware engineering teams. The software team should have been able to identify that the wrong valves were being actuated.