Basic Trope: A character claims to have a particular Achilles' Heel, but is lying.
Straight: Before storming the castle of the evil Lord Bob, Alice hears that he has a weakness against mirrors. She arms herself with a mirror and charges into his throne room, only for him to merely stroke his chin and say, "Hmm, I think I need a shave."
Lord Bob, so it is said, is reduced to a quivering mass of jelly if a mirror is within fifty feet of him. In fact, it's regularly featured in local newspapers.
Lord Bob pretends to be a vampire, and he even sets up fake mirrors to make it seem like he has no reflection, but he only pretends to have weaknesses to garlic, direct sunlight, silver, wooden crosses, and holy magic.
Alice has only heard a rumor of Lord Bob's weakness against mirrors, so she only packs one just in case, and it turns out to be about as useless as she thought it would be.
I Am Not Left-Handed — literally, Alice has aimed her attacks at Lord Bob's left hand, but it turns out he's right handed.
Justified: Lord Bob used to be weak against mirrors, and has since recovered from this weakness, but does nothing to defy the rumor.
Inverted: There is a rumor that Lord Bob draws his powers from mirrors. Alice breaks every mirror in his castle, only for Lord Bob to reveal that his Fake Strength Source was a lie.
Subverted: At the midway point, Alice discovers that Lord Bob's supposed weakness against mirrors is nothing but rumor and speculation. But then she finds out that he does have a weakness to a special Magic Mirror. Her quest then revolves around finding this particular mirror and using it to take Lord Bob down.
Double Subverted: At least, that's what he lets Alice think, so that she gets close enough that he can deliver the killing blow.
Zig Zagged: ???
Averted: Lord Bob doesn't spread any fake rumors about his weaknesses.
Enforced: The show's producers have the odd idea that it could become seriously harmful somewhere down the line for children to believe in unrealistic Weaksauce Weaknesses, so they want to present An Aesop about not believing in that sort of thing.
Lampshaded: "You really thought I would be weak to silvered glass?"
Invoked: Lord Bob has been spreading the rumors of his mirror-weakness intentionally, after he witnessed how many overlords were defeated when word got around of their Achilles Heels.
Exploited: Alice walks into Lord Bob's throne room with a mirror and a look of smug self-confidence, to distract him from the great big machine gun she's got behind her back.
Played For Laughs: After finding a newspaper declaring that Lord Bob loses his powers when confronted by a mirror, Alice cleverly sets up a Funhouse of Mirrors right outside the door to the castle — "Evil Overlords Get in Free!" in an attempt to lure in Lord Bob. He enters the Hall of Mirrors, and Alice springs her trap! ... but he fails to pay attention to her In the Name of the Moon speech, as he keeps going "What was I thinking? Ebony and olive are horrible color choices for me!"
Played For Drama: Alice has decided she must take down the unstoppable tyrant, Lord Bob, or die trying. Her grandmother's dying words suggested that a mirror might, just might, be able to counter the wicked sorcerer. After many brave and terrifying adventures, Alice makes it to his castle and challenges him to single combat. He disarms her and defeats her easily, and when she pulls out her grandmother's mirror, her last-ditch hope — he laughs, plucks it from her hand, and breaks it without a thought.
Untwisted: Alice hears that Lord Bob is weak against mirrors. She pulls out a mirror when she charges into his throne room — it works!