Programming

git gets easier once you get the basic idea that branches are homeomorphic endofunctors mapping submanifolds of a Hilbert space.

(source)

Edit: but to actually have content in this comment, I'm not sure the mental model is the problem. It's not that alien that a good explanation wouldn't help, but it took a long time for git to start paying any sort of attention to "human readability." It was and still is in a way "aggressively technical" and often felt like it purposefully wanted to keep anybody but the most UNIX-bearded kernel hackers from using it. The man pages were rarely helpful unless you already understood git, the options were very unintuitively named, etc etc. And considering Linus' personality, I'm not exactly surprised.

With a little bit of more thought on how to make it more usable right from the start, I'm not sure it'd have such a reputation as it has now. The reason why I think this endofunctor joke is so funny is that that sort of explanation to "simplify" git wouldn't have been at all out of place – followed by the UNIX beards scoffing at the poor lusers who didn't understand their obviously clear description of what git branches are.