A Japanese surprise attach sought to wreak havoc in the United States with ballooned explosives. In theory, the balloons (over nine-thousand of them) would ride the jet stream for a few days before releasing a payloads of explosives across America. In reality very few balloons are estimated to have made it to the United States and only six deaths resulted from the attack, all from one balloon. Many more were found detonated.
Type 94 Nambu
The Imperial Japanese Army favored this light-weight pistol at first, especially pilots. But the unique design, which featured the sear bar on the outside, meant it could misfire very, very easily, so much so that urban legends persist of soldiers using this defect to their advantage for surprise attacks.
This German pistol was made for self-defense, not war (and in fact production of the weapon stopped on when WWI broke out). Known now for being the world's smallest pistol, it fired six rounds with no recoil. Unfortunately the bullets themselves were incapable of causing much more than a slight sting.
New Zealand's first tank design was born of desperation, and sadly the impressive look left much to be desired in terms of actual function. At its core it was a tractor, giving it little speed compared to other tanks. The armor was flimsy and easily pierced by standard firearms. Finally, the guns themselves were in fixed positions.
The idea is simple enough: take the beloved concept of the bayonet and transfer it to the pistol, making it what would amount to a dagger. But the firing mechanism in early models was easily upset, so swinging the pistol itself could cause it to misfire, leaving you exposed given how long it took to reload (assuming you didn't shoot yourself).