The title made me think about what my answer would be. If there's one thing a programmer should know, what is it? What is the most important thing I didn't understand about programming when I started doing it?

My answer would be: that code should be seen not as a static thing, like the answer to a math problem, but as an evolving effort to figure out the right question to be the answer to; and that it should thus be written to be easy to change.