Ask HN: How do you get better at spotting Unknown Unknowns?

The only way I have ever known is to have the idea that every single decision is imperfect in some way, and to keep a low resolution idea in my mind of what kind of imperfection that might be and what consequences it may have. If one possible imperfection nags at me for long enough, then in thinking about it, the resolution can be increased and a determination can be made about its impact, and whether it is an acceptable compromise. This tends to happen preemptively, but can also be used when a concerning unknown begins to rear its head.

This is not as daunting as it might sound. It's somewhat similar to how a "picky" personality would think, e.g. Jerry Seinfeld's character in his sitcom. You have to be an annoying nagger in your mind, albeit without the comedically hyperbolic verbalization, obviously.

This can make you much slower at first, as you'll be debating things in your head a lot. But I find you get a lot faster once you get used to it and the "nagging" happens in a not-very-high-effort thought process.

tl;dr: perfectionism.

Of course, for specific situations, there will be known tools and processes for flushing out unknown unknowns e.g. fuzzing.

Mapping of known knows and known unknowns with comparison to previous documentation on similar projects, experience and educated guesswork where answers are not forthcoming.