What should every developer know?

C.

Design and UX specially

Everything and 42.

What do you mean by know? Are you referring to languages and technologies, or are you talking about... refactoring, planning and things that are not strictly related to development? Because I would love to talk about either, but one is definitely shorter to write. :)

Tests.

Debugging strategies: Eliminating variables and addressing your own assumptions about the system in order to debug effectively. I think this fundamentally applies to all subsections of our industry.

that php, javascript and java jokes never get old :)

Write maintainable code from the start.

I'm saying what some would assume is obvious, but I'll say it anyways. It's OK to ask questions. No one knows everything. What you should do though before asking your questions is do your due diligence. Step through the code, check the logs, Google the error etc.

Someone more junior would perhaps due less of this, but as you progress in your career your diligence should increase.

Part of being a software developer is being a detective. 🔎 Now crack the case! 😉

Every developer should understand how their tools work. That is, OS, IDE, text editors, shell/command prompt

How to google stuff :)

git

They should know the seven commandments and keep them in the drawer for easy reference:
Snickers, Toblerone, Ritter Sport, Nestle Crunch, Almond Joy, Kit Kat, Milky Way

How to talk to people. This has become a dark art.

How a computer works.