Many unusual inventions never worked out.
In the nineteenth century, the passengers of the balloons Skylark and Flying Cloud discovered an unusual sonic effect that became known as the balloon telephone.
Because of resonance in their balloon envelopes, the occupants found they could talk across large distances. Voice communication was possible across a mile of clear sky.
It was one of many discoveries that could have been patented, but somehow never were; probably because they had no apparent use. A slightly more useful never-patented invention was the geostationary communications satellite.
The following invention probably falls in the first category:

Transvisible image projection: a new display methodology (patent pending).
Designed to increase situational awareness, the user will need to wear a virtual reality helmet. The exact position of their eyes and the target has to be known at all times.
Powerful software can make every other object in the user's environment transparent to a selected target object, which is being tracked by stereoscopic cameras.
If someone was coming their way down the street, they would see a small out of focus figure projected through a wall or a table, steadily getting larger and more in focus.

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Probably the best hard SF novel ever written: Infinite Thunder by Jack Arcalon.
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