Segway. They managed to keep it secret until the official announcement, and at the time you wouldn’t have believed it if you hadn’t actually seen it (video of it on network news, that is).
I remember a slightly different story: There were claims they would revolutionize urban traffic. Then they came up with a rather ridiculous solution, especially in consideration of the price point. What's left are tourist tours for people who don't want to walk and who would rather prefer sitting in front of a TV set.
The Segway was predicted to become what the electric scooter has instead.
An ironic twist that the company that bought the Segway brand makes nearly every one of those scooters. I guess it sort of worked out after all?
Price point matters.
And people realized cities aren'r barrier-free. There are lots of stairs and similar obstacles that not only cannot be navigated on a Segway, but the Segway is much too heavy too easily carry it over those.
Well Segway itself may not have succeeded, but I see more and more people roaming the city on self-balancing scooters.