Night Goat
The Immovable Autism
Mordenkainen's Disjunction from Dungeons & Dragons, a 9th-level spell that permanently disenchants magic items. If a player character uses it, he destroys the loot and the rest of the party hates him. If a villain uses it, he destroys tens or hundreds of thousands of GP worth of items and leaves a player character incapable of dealing with level-appropriate challenges. There are a lot of useless spells in D&D, but this one stands out because it's a 9th-level spell that's bad for the game no matter who casts it.