For me, it was Google Glass. But maybe I was just naive.
When it was released I saw it as the future of computing. The key tech lead on the project gave a talk at ETH to a full auditorium, just before it became publicly available. Needless to say it blew my mind. Speech recognition, plus a very practical looking augmented reality display, plus a first-person camera. I imagined some really cool computer vision apps becoming available soon that would integrate this new type of computer more closely with humans. This could be a unique gadget that would be able to see and understand the context in which we requested information from it.
I still have a hard time believing* it failed for the reasons it did. Although some say it was inevitable. Well, hindsight has 20/20 vision!!
That failure makes me a bit skeptical about devices like Google Tango, Microsoft Hololens etc. I think that story was an important educational experience for my generation of geeks. It's not the first time something like this has happened, think about the Palm computers of late 90s, and probably not the last either. Jobs' ideology of telling the people what they want instead of asking them doesn't always work!
* I am probably extra biased because my Master thesis tried to do 3D scene understanding including activity recognition from a much clunkier head-mounted camera. And seeing the device made me think I could've produced a very practical demo if Glass had been available back then.