Tutorials are great for syntax and concepts, but there's a whole category of things you only pick up once you're working on a real codebase with real constraints and real teammates.

For me it was reading other people's code — specifically, old code written under deadline pressure with no comments. Tutorials always show clean, well-structured examples. Production code is... different.

What's yours? Could be technical (debugging, testing, architecture) or non-technical (communication, estimation, requirements that change halfway through a sprint).