As a car enthusiast, I can think of a good one, the Ford Nucleon.
During the 1950s and 1960s, there was considerable interest in nuclear power and its potential applications. This led to the idea of using nuclear energy to propel cars. The concept behind a nuclear car was to utilize a small nuclear reactor to generate steam, which would then power the vehicle's engine.
Of course back in those days, this was extremely futurustic and some at the time thought this would be a game changer, but ultimately, the safety aspect was one of the biggest reasons why this idea was dropped, and I probably don't have to explain why it may not have considered to be safe, I mean, it was using nuclear power, so even if the engineers tried to make it as safe as possible, IF something went wrong, it would have been catastrophic.
Ever since then, the interests in the automotive sector has shifted to Electric and Hydrogen.
Still, a very intriguing concept car and idea.
Outside cars, you have blimps, and I personally believe if we tried to make something like a hindenburg today with existing technology, we might have been a lot more successful than back then (as it goes way back to 1930s), there are still some blimps used occasionally, I also don't believe those use hydrogen(?), but they are not the "game changer in air travel" it was once seen as, although we can't rule out a comeback.
What about you guys?
Google Wave
The Commodore Amiga was superior to Mac and PC when it came out but unfortunately for the engineers, the business was run by cretins
Dvorak keyboard layout, maybe
I wouldn't call the Amiga a flop, it just didn't survive. It was reasonably successful for a while.
I miss Google Wave. It was my preferred way to collaborate with friends for a long time.
Wave was simply ahead of its time and made by the wrong company. Google never supports anything it creates long enough for it to establish a path forward. Now, people don't support Google products much because they know Google will cancel it within 3 years.
I can simotanously acknowledge and accept two things when it comes to Dvorak.
I'll learn Dvorak when everyone else does. Same with Esperanto.
The rest of the works keyboards are QWERTY by default. Dvorak is everywhere too though. Just a few clicks in Linux, macOS or Windows and you’re set up.
That said, I learned it because I had RSI at the time. 20 minutes a day for about three weeks (when I was young and my mind was absorbent) and I was almost at my original typing speed.