There were a lot of pre-war/early-war weapons that came up short during the war, but I’m not going to count those.
There were the German “Wonder” weapons that came late and had unreasonable, even fanciful, expectations, let’s skip those. I’ll even include in those, things like the King Tiger, which, when they weren’t broken down, didn’t really produce proportionally to the resources used to build them.
There were weapons that underperformed until they were updated (Sherman, P38, Mustang).
Japanese submarines were among those which were simply poorly utilized.
What we want are weapons with reasonable expectations which just came up short.
My first nominee is the Messerschmidt (x)10 series of planes. The Bf110 was the first real attempt at a heavy, “convoy fighter”, designed to escort bombers. Turned out that it couldn’t keep up with the single-engines fighters sent up against it, and so actually needed fighter escort of its own. It might have been better at ground attack, but not as good as the Ju88, so it didn’t really find a new niche there. It did serve somewhat usefully as a nightfighter.