This might not qualify because it was never really in the zeitgeist, but I remember around 2000 that flywheels were going to be an amazing mobile energy storage mechanism. Your car would rev up a massive disk inside a vacuum, and use its momentum to propel itself in stop and go traffic.
I believe the showstopper was that in even the slightest fender-bender, the flywheel would get off-balance and yeet off in a random direction, causing mayhem and calamity.
I believe the showstopper was that in even the slightest fender-bender, the flywheel would get off-balance and yeet off in a random direction, causing mayhem and calamity.