The Bare Minimum
There are many tools that I use that I don't know how to use. I don't mean this literally; I can use them, but not well, and certainly not to their fullest extent. This is not good.
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From Scratch
My best lessons have come from building a complex piece of software by myself. When you implement something, you hold a better understanding of how it works, and how you can apply that knowledge to other systems.