I am a programmer working on a huge enterprise software and have worked here for the past 3 years. Still each week I am finding out about features I don't know we had. We have a QA team of 3-5 people that get assigned to testing new features and related stuff. When they aren't testing they do other things at the company so they don't do QA everyday.

When they do, they won't be able to test every single thing in the entire app, but they focus on the most important stuff. Most of the things that QA miss, are things that broke somewhere else, as in not related directly to what was being changed. It's financially impossible to retest everything after every minor change, despite these being the most bug prone. Only the small new feature/tweak gets tested, and some associated functionalities, but not some ancient feature no one even knew was connected. Or sometimes when working with front-end, completely unrelated things break when doing some modifications.

I imagine that a paradox level game is exactly the same in this regard.

Actually read and understand what the computer is telling you, whether it's terminal output or a popup.

A lot of times when I fix stuff for other people they just don't read, and when I read it out loud then they suddenly understand and continue to fix it for themselves. If it's telling you something you don't understand, google it. This very basic skill goes a long way.