Useless Useful Spell/Playing With

Basic Trope: A spell or power appears useful at first glance but isn't worth using and/or doesn't work when it's needed anyway.
 * Straight: The enemies you'd want to hit with Instant Death are immune to it.
 * The Instant Death spell almost never hits, or is extremely costly to cast compared to normal spells.
 * Exaggerated: There is only one enemy in the whole game who isn't immune to Instant Death, and there's easier ways to deal with it anyway.
 * Justified: The Instant Death channels the power of the Big Bad, so the bosses, who are his lieutenants, are immune to it.
 * Inverted: The Magic Missile spell seems useless at first glance but turns out to be a staple of the gameplay.
 * The normally Useless Useful Spell is the ONLY way to defeat a specific boss.
 * Instant Death is a Disc One Nuke capable of obliterating 90% of the game's enemies with great ease.
 * Subverted: The Instant Death spell almost never hits... but there's a spell that guarantees the next spell to hit...
 * Double Subverted: The Big Bad is immune to that one too.
 * Parodied: The character uses the spell against a known invulnerable enemy, for the fifth time.
 * Deconstructed: Protection against the Instant Death spell becomes so commonplace that the spell itself is forgotten, and in time, so are the countermeasures.
 * Reconstructed: The final boss is ancient enough to remember using it too.
 * Zig Zagged: The Instant Death spell has a 50% chance of doing nothing, a 10% chance of healing the enemy, a 10% chance of killing a member of your team, a 10% chance of conjuring a cloud of fluffy kittens, and a 20% chance of actually working. And that's just against normal enemies.
 * Averted: The Instant Death spell has a flat chance to work on anything, making it a Game Breaker.
 * Alternately: There's no Instant Death spell, but lesser status effects work regularly on the majority of enemies and are actually useful.
 * Enforced: "We can't have the player one-shotting the Big Bad with Instant Death!"
 * Lampshaded: "Wouldn't it be nice if we'd meet something that Instant Death would be good against?"
 * Invoked: The Big Bad invented that spell to exterminate non-casters, and made sure it could be blocked by the slightest magical protection.
 * Defied: Before fighting the Big Bad you secretly replace his Anti-Magic ring with an ordinary ring, when you fight him later he has no protection against Instant Death.
 * Alternatively, you're playing a Pokemon game.
 * Discussed: Someone suggests just using the Instant Death spell on the Big Bad, but another party member points out that it will never work.
 * Conversed: Two Gamers on a Couch are playing EverTropes. The guy playing the fighter makes fun of the guy playing the wizard because Instant Death doesn't work on any of the enemies. Wizard reminds his friend that it works on him.

Back to Useless Useful Spell, except it doesn't work when you want it to.