[reader_1000]

I agree, any direct / field modification should be assumed to be not-thread safe. OTOH, I think Go made a mistake by exporting http.DefaultClient, because it is a pointer and using it causes several problems including thread safety, and there are libraries that use it. It would have been better if it were http.NewDefaultClient() which creates a new one every time it is called.

[unscaled]

I think the original sin of Go is that it neither allows marking fields or entire structs as immutable (like Rust does) nor does it encourage the use of builder pattern in its standard library (like modern Java does).

If, let's say, http.Client was functionally immutable (with all fields being private), and you'd need to have to set everything using a mutable (but inert) http.ClientBuilder, these bugs would not have been possible. You could still share a default client (or a non-default client) efficiently, without ever having to worry about anyone touching a mutable field.