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Maybe not exactly what you're looking for, but in the movie Iron Man 2, War Machine was outfitted with an extremely powerful rocket described thus:
"This is a kinetic-kill, side-winder vehicle with a secondary cyclotrimethylenetrinitramine RDX burst. It's capable of busting a bunker under the bunker you just busted. If it were any smarter, it'd write a book, a book that would make Ulysses look like it was written in crayon. It would read it to you. This is my Eiffel Tower. This is my Rachmaninoff's Third. My Piéta. It's completely elegant, it's bafflingly beautiful, and it's capable of reducing the population of any standing structure to zero."
When he attempts to use it against Whiplash, it simply bounces off of him without exploding and fizzles on the ground.
That is a great example, fairly close to what I was after. However, I was looking for examples where the weapon fails due to it being the wrong choice of weapon rather than user error, manufacturing fault or delayed effect. Like shooting a bear with a specialist hunting rifle with armour piercing rounds, and the bear hardly even slows down.
Edited by Cheeser
How about Cross-Melting Aura? You grab a cross to ward off an evil monster, expecting Holy Burns Evil, and instead the evil monster pulls a Scissors Cuts Rock and destroys the cross.
If not for this anchor I'd be dancing between the stars. At least I can try to write better vampire stories than Twilight.
That's interesting... I think I was looking for examples of a False Prophecy but for a weapon rather than a person. Thank you anyway.
Holy Burns Evil averted or subverted or something. Not sure which but it would go under Holy burns evils, and start with “Suberted/Averted(Whichever it is) in [Example]”
EDIT: I take it back, this is wrong. Read your post more closely and saw you didn’t mean faith based objects.
Edited by AgentAndrew0
I'm looking for examples in films (or TV) where all the good guys have faith in a special weapon (silver bullet, wooden stake, mystical amulet... etc) but during a confrontation, the weapon is revealed to be useless. I can think of examples where it doesn't work and then it does (Fright Night, Transformers: The Movie, Never Say Never Again) but not one where it simply fails. There must be a few.