The thing that impressed me the most was the reserved identifiers rule in C, I didn't even know that existed and wow it's worse than I though, it's so ill-thoug

The thing that impressed me the most was the reserved identifiers rule in C, I didn't even know that existed and wow it's worse than I though, it's so ill-thought-out that it covers most generic words you can use in your code, I wouldn't be surprised if >90% of the existing C codebase out there is already violating it unaware. But just like most pitfalls in the language, it's a consequence of the early developers not putting much thought when assigning "namespaces" on the C library functions/macros. As for the locales non-standard behavior in GNU tools, it's no strange that GNU project always historically deviated from POSIX standard.