tl;dr: Read the effin’ source!

If a function or a parameter isn’t documented on the Codex, that doesn’t mean it doesn’t exist. Look for it in the source code. Doubly so for actions and filters.

When a template tag doesn’t behave properly, don’t just jump over to WordPress StackExchange. Dig up it’s definition and try to figure it out for yourself first.

If your job title includes the word “WordPress” + any of “developer”, “rockstar”, “ninja”, “mutant turtle on steroids”, jumping into Core code should feel as natural as breathing. Set up your development environment so that it only takes a few keystrokes to do.