You could probably nail someone with a lightsaber or a phaser on possessing an improperly labeled laser device unless you had the foresight to put on a sticker saying something like “Warning, Class III device, may cause extreme eye damage (and your head will fall off)”.
The scariest science fiction weapon I can think of is razor floss. Consider an invisibly thin unbreakable monofilament. If you walk into it, you wouldn’t even know until your torso fell apart. Throw a tangle of it at an enemy and it’s game over.
And as far as I can tell, it would be totally legal. Hey, it’s just string.
It’s just string in the sense that a knife is just a block of metal. Maybe, but it’s a sharp block of metal. I expect that any court presented with such a case would rule razor floss to be a kind of blade, and thus subject to whatever local rules apply to blades. Given that it’s inherently concealed by nature, and that it’d have to be longer than 4 inches to be effective, I think it’s safe to say that it’d be illegal most places in the US.
Generally, the “possession of a dangerous weapon” thing covers any contingency where the device appears to be used in a threat. If too many people are carrying around your proto-phasers or monofilament, and it proves impossible to simply charge people after use, too many incidents or one big high-publicity incident, or just general establishment paranoia, then the “there oughta be a law” mentality kicks in and the specific law is added to the books.
IIRC it was simply paranoia that got nunchuks and throwing stars specifically made illegal weapons in Canada.