Ask HN: What should every programmer know?

I am about to graduate college, I have had an internship for almost the past 3 years as a software engineer and prototyper, mostly working with Java. I am about to look for a full time job and was wondering what should every programmer know? This can be anything from web development, to requirements, or to database entry. So that way I can be prepared for an interview and become a better programmer. I looked online and wanted to know what you guys think.