One invention for the sake of making the filming easier in Sci-fi shows like Star Trek are gravity webs, meshes, or floors, that provide gravity in environments like space that shouldn't have any. This makes it easier to film, because the "space" environment can be shot in a basement in hollywood, instead of having to actually take the actors into low earth orbit, which would be significantly more expensive.
Such a device would have massive utility. Not only could you walk in space instead of float, and travel between the stars without your bones weakening into goo, but you could exercise with them (the gym has DOUBLE GRAVITY today), make transportation easier (the packages now weight NEGATIVE whatever because there's a reversed gravity mesh in the truck's ceiling), or just carnival style novelty (as in, hey, let's walk on the walls! Because we can!).
However, it can't happen. The best way to prove to me that it can't happen is to build a perpetual motion machine with it, so I'm going to do exactly that. Please enjoy this flagrant violation of the laws of thermodynamics.
As you can see in the diagram, the very large ball falls onto the turning wheel, imparting the energy it got from gravity and thereby providing the extractable energy. It then falls to the lower ramp, moving to the other half of the device. At this place, gravity is reversed, so it "falls" to the top of the machine, where it hits the ramp that moves it back to the other side. Oh hey, gravity is reversed again due to the absence of the mesh in this part, so it falls again, back to the wheel. This would endlessly produce power, against all reason.
So obviously the gravity meshes either have to use more power to function than all the falling that happens in their area, or they just can't happen. In either case, Sci-Fi fans everywhere pout at this rude intrusion by reality. Then they go back to enjoying their fandom, because since when does television, movies, or books have to in any way correspond to reality? Wait up, guys.
Thanks to D. K. Wolfe for the illustration.