The complexity of USB C, with a diverse range of features supported (or not) by different cables that can't be visually distinguished: https://www.extremetech.com/computing/257684-usb-c-chasing-unachievable-goal
miniblog.
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JSON is too small (no comments) and YAML is too big (many string syntaxes, relatively few implementations).
TOML is in the sweet spot for complexity, but I agree this syntax is by far the most confusing part.
TOML 1.1 improves it at least:
"Minus 100 points", an article on deciding how to add features C#, remains one of the best introduction to PL design principles I've seen: https://docs.microsoft.com/en-us/archive/blogs/ericgu/minus-100-points
(Design is hard, combinatorial complexity grows easily, saying "no" needs to be a default.)
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Writing a REPL that evaluates-as-you-type, keeping the UI responsive and staying defensive against runaway memory usage: https://scattered-thoughts.net/writing/making-live-repls-behave/
It's a really hard problem for PLs, but even this small live demo has a lot of complexity.