Widespread commercial nuclear fission power, for better or worse.

Thanks to the gross mismanagement of Chernobyl and Fukushima and over-reaction to it (EDIT: and also Greenpeace, see reply), it has been largely defended and derived. And thanks to renewable energy being more affordable than ever, it might no longer make sense to build it.

That said, flying cars and generative AI have one thing in common: they are more harmful than anticipated.

If we had flying cars, even if the crash problem was solved by making them autonomous and fully protected from cyberattacks and unable to perform terrorist attacks (good luck lmao), we would still have noise everywhere. Think low-flying planes, but constantly. (And, of course, the proponents of the technology would call everyone complaining about it Amish Luddites who want to "stop progress".)