Gosling's Emacs was scripted in MLisp, which was a remarkably quirky language: https://news.ycombinator.com/item?id=8727085
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Today I learnt that Emacs 28 shipped a context-menu-mode! https://oylenshpeegul.gitlab.io/blog/posts/20230129/
This seems quintessentially Emacs: deeply hackable, but building UI features in an order very different to the mainstream.
It is remarkably hard to escape command line arguments safely on Windows, and the standard libraries of multiple languages have needed patching: https://flatt.tech/research/posts/batbadbut-you-cant-securely-execute-commands-on-windows/
Claude asked me a question today: was I looking for an Emacs plugin (because I was talking about elisp) or a Rust program (because I have configured Rust preferences)?
I'm really impressed, it's rare to see LLMs ask follow-up questions.
(I wanted Emacs in this case.)