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MiniDiscs. So much better than those bulky cds...
Segways had a ton of hype and were going to revolutionize the way people get around. Now I just see them being used by tour groups and mall cops.
Second Life. EVERYONE was going to be there.
The Titanic. (the movie did ok though)
Are you talking about fyre festival?
The 1970s Dale, a three wheeled car which was advertised as doing 70 MPG.