Computer algorithms in human readable format
Hold on! Before you call me crazy, please let me clarify. Yes, I am going to tell you how to handle C++ exceptions in C code, but don't worry, I know this is a terrible idea, and (hopefully) no one is actually going to do that.
In the field of computer science, the Paxos algorithm is notorious for how difficult it is to understand. I had to learn the Paxos algorithm in my distributed systems class. I even have "implemented" it by translating Leslie Lamport's TLA+ to Python. But I didn't understand it until much much later.