Designing and writing explicit state machines. They seem to come up more in low-level code than high-level code. I prefer to generate them, so I would add to

Designing and writing explicit state machines. They seem to come up more in low-level code than high-level code. I prefer to generate them, so I would add to that "tools for generating C code and state machines". (Tangent: A guy who worked on Verilog and FPGAs told me that the "whole idea" is state machines... and they are hard to get right.)