Hmmm... There's probably a million things I've never used in a game, but I managed to think of one in particular.
The flamethrower in Extermination.
Check this out. For those of you not in the know, Extermination was a fairly-original survival horror game featuring you as a sort of operative being dropped into a research base in the Antarctic. Why there? Because some remnant of an alien hit the Earth and had to be put in cold storage to prevent contamination. Guess what happens.
So yeah, the whole place is literally infested. This stuff infects living beings, machines, the walls and floors... It's more contagious than The Thing. A feature of the game is your infection meter, in fact. Get too high on the meter and you're dead. Still, this isn't the point of the post, is it? No, this is about the flamethrower attachment for your assault rifle, which is customizable.
So, they give you this attachment to allow you to spew fire, and then you find out something that's...uhhh, rather important. These creatures and their infectious cells thrive in water, any water. But more than that, the hotter the better! Exactly how will le flamethrower help, hmmm? I tried the sucker out. Lord knows I tried. I killed exactly NONE of the creatures with fire. They didn't seem perturbed by it at all. Bullets, of course, were totally effective. Why give me fire in the Antarctic? THEY HATED THE COLD, DAMMIT!