[bm98]

To me this is the crux of the issue. And it's non uncommon in software development:

The developer fails to check for an error condition or raise an exception because doing so would add too much complexity to the system. So instead it is assumed (or hoped) that it simply can't or won't happen. Problem solved...

(Edit: Or that the user will just have to reboot if it happens.)

[ReGenGen]

Yes. Not checking for errors is the feature of the MCAT "solution" which hides the system from pilot's training AND involvement with other 727 system/team engineers. A bureaucratic solution to a marketing problem. I wonder how many programers they had to go though to find it.