After using Rust for many years now, I feel that a mutable global variable is the perfect example of a "you were so busy figuring out whether you could, you never stopped to consider whether you should".
Moving back to a language that does this kind of thing all the time now, it seems like insanity to me wrt safety in execution
Global mutable state is like a rite of passage for devs.
Novices start slapping global variables everywhere because it makes things easy and it works, until it doesn't and some behaviour breaks because... I don't even know what broke it.
On a smaller scale, mutable date handling libraries also provide some memorable WTF debugging moments until one learns (hopefully) that adding 10 days to a date should probably return a new date instance in most cases.
Moving back to a language that does this kind of thing all the time now, it seems like insanity to me wrt safety in execution