A phone from the year 2039. Due to improvements in battery capacity, it holds enough energy to serve as a rudimentary laser pistol. But that's not why it's dangerous. Go ahead, ask it how many networks it can access simultaneously. It's not a general purpose AI. It's just a very competent personal assistant expert system. And it's backwards compatible with systems we haven't even invented yet. In the year 2019, this thing could bring half a city to its knees just by turning it on.
KARR, the prototype for KITT from Knight Rider: a self-aware, self-driving muscle car with one directive - self-preservation.
A crystal ball, forged by the mightiest of elf-smiths long ago, granting its master vision across their land, and mental communion with other holders of such balls. It's called a "palantir", whatever that means. Kinda like Google for wizards, right?
Portable teleportation gates or portals - specifically ones where the gate stays open and there's no barrier of force field over the entry way. On the surface they seem fantastically useful - the super rich use them to have a single house that has rooms in different climates an locations around the world. But...
What happens when someone puts one portal at the bottom of the ocean? Or, worse, on the moon?
A pressure cooker, built with a fake safety valve and a fake manual valve. The only way to release the pressure is to open the lid, which you can do only after having turned the gas off or having removed the cooker from the burner. But if there is nobody around to do that...
If you have a mass that itself reacts counter to gravity, but which regular matter objects feel in a normal gravitational way, then you can make a perpetual motion machine that accelerates forever. You take the anti-gravity mass and an equal gravitational mass, let them go, and watch. The regular gravitational mass falls toward the anti-gravitational mass, which itself falls away from the gravitational mass. One falls faster and faster. The other anti-falls faster and faster. So the system continues to accelerate together and will reach crazy velocities with no outside energy input.
Nigh-indestructible gloves. They were created for industrial uses but allow thieves to bypass some scanners and most physical traps.
Insulation foam meant for transportation of fragile goods. A lot of it can break most walls and will suffocate anyone engulfed. The solution that dissolves it turns the foam into harmless gas and thus potentially hides its uses.