The world in which IPv6 was a good design
One big UX mistake of IPv6: it was not made backward compatible with IPv4. (v6)0.0.192.168.1.10 == 192.168.1.10(v4).This simple design when planning and rolling it out would have meant incrementally updating the networking stack to also support v6. Now it turns out v4 and v6 are completely different, and no one has a big enough reason to make the change until everyone else makes the change. Hard chicken-egg problem.