Comment by Jonathan Cutrell on "What rookie mistakes have you made as a beginner, which you have improved upon now?"

Perpetuated the quick-fix mentality. This is really quite terrible, and a cycle that is hard to break from. Start with tests, code reviews, and continuous improvement on day one, and don't break away from that.
Believing technology was inherently valuable on its own. (It is not.) The people you encounter in your career who are not developers (read: project management, executive teams, clients) will likely not understand the value of testing, or the value of new technology for that matter. The more you understand how to translate value into terms that others can understand, the further in your career you will go.
Not automating repetitive tasks. I don't know how much time I've lost, but every task you repeat that isn't made more efficient is time lost. See:
Understanding my own values, goals, and clear path earlier. If you don't set yourself up for longer-term career fulfillment, eventually you will hate your job, no matter how cool it is or how high-paying it is. Find what you care about. Find your underlying values and career purpose. It is the fuel you need.

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