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?
I remember they tried to use the environmental angle for marketing. They claimed that they were making use of discarded fruit pulp that would have gone to waste.
But of course it was actually an efficiency nightmare. They shipped the pulp to their factory, then shipped the weight of the pulp plus juice to the customer, who would then throw out half the weight of each package.
It would have been way more efficient for them to just buy the pulp, squeeze it in industrial quantities and sell bags of juice like some trendy health thing. But of course then they would have been a juice company instead of a tech company, and juice companies don't get as much venture capital.