Floating point numbers behave in ways that surprise developers until you understand the underlying representation. Once you do, the behavior is predictable and the bugs become avoidable.
The concrete differences between processes, threads, and coroutines at the OS level, and how to choose the right concurrency model for the problem in front of you.
What the stack and heap actually are, how memory allocation works at the hardware level, and why understanding this makes you a better programmer in any language.
From a line of source code to electrons moving through silicon - the mental model every engineer should have.
The compiled vs. interpreted distinction is blurrier than you think, and understanding the spectrum explains tradeoffs you make every day.
