I'm confused what you ascribe to as undefined behaviour. What does that mean in the Go context? There is no mention of what UB is in Go at https://go.dev/ref/spec .
With a race of a non-trivial object in Go you're violating assumptions of Go's language runtime. Physically impossible things can't happen because that's what physically impossible means, but stuff which is merely written down in a document like the one you linked is fair game for such problems.
In terms of concrete examples, this might allow remote code execution, arbitrary reads or writes of memory that you otherwise don't have access to, stuff like that.
