Lion's Eye Diamond

A famous design mistake that became one of the most surgically abused cards in the game's history. The intent was a Black Lotus you had to empty your hand to fire, a clean drawback that priced the free three mana: you discard everything before the mana arrives, so a naive player gets a worthless burst with nothing left to spend it on. The break is in the gap between when a cost is paid and when a spell on the stack resolves. The activation discards your hand as a cost, and because it is a mana ability it resolves immediately without ever going on the stack. So the entire combo industry learned to time the crack to that window: cast a spell, let it sit on the stack waiting to resolve, then activate the diamond in response so the discard feeds whatever that spell is about to do. Storm decks treat it as a ritual that also stocks the yard for Yawgmoth's Will. Reanimator strategies pair it with non-targeting recursion like Exhume: cast the spell with other mana, hold priority while it waits, then crack the diamond to pitch a fatty into the graveyard the spell is about to plunder (a targeting spell like Reanimate cannot do this, since the creature has to be in the yard before it is cast). The "Activate only as an instant" line is the restriction the designers leaned on to keep it honest: it forbids activating the ability mid-cast, so you can never pay for the hand-cast spell with the diamond's own mana. The whole power level lives in that priority window, which the original designers did not foresee. It is the canonical example of a drawback that becomes an upside the moment you read the rules for when costs are paid.