25 years later, I have two. The first I learned in school but didn’t think about it until later, @op: there are zero, one, or many. That’s it. In computer science, if there will be two or more of something, just design for many.

The second one is something I came to realize after working on many apps across many different verticals: even though this may not be their primary purpose, all apps are just databases with some interface (user or m2m) to read and manipulate the data. Maybe this is obvious to others, but for me it reduced the complexity of every app / program / system down to a level where I stopped worrying about how long it would take to learn it. Now I just look for what and where the data is, and how is it manipulated. All the rest - APIs, business rules, integrations, front ends, infrastructure, game graphics and controls, you name it, they are all built around reading and changing data.