I built a competitor to Reddit and Digg (at the time) called RatherGather that had some differentiators that ended up not mattering to people. I wish I understood Metcalfe's Law better at the time.

I built a mobile game called Roaring Racers where you had to use your voice to make your car move (roar louder, go faster).

I built a stickers app for iOS stickers launch that allowed you to import anything. This one wasn't a total dud, but the way Apple positioned the store, the traction wasn't at a scale that made it worth my time.

And then of course there's the laundry list of companies that I've been employed by and built products for where months into it I discovered nobody in the company had really done any market testing, but they got clearance to build something for $XXX,XXX anyways.

9/10 startups fail, so just by nature of that fact, most of us spend our time working on building tech that never really "makes it".

There have been great successes too, but you asked about the failures.

Edit:

I forgot one that is fairly recent too. I built a Windows app that allowed you to watch Twitch picture in picture while playing a game. Sold one copy before shutting it down.

I absolutely love your creativity and persistence. Thanks for sharing. You're totally going to hit one out of the park.

Roaring racers sounds like a brilliant idea! Where can I play it??

It came out maybe a year after the App Store launched, and the 100 copies I sold never justified maintaining it. It's long gone. Only way to play it would be an old iPhone that still has it installed and I don't even have one of those.

Good idea but I for one am glad that roaring racers did not become pokemon go. Kids are loud enough.