Flash

Almost nobody has ever cast this the way it reads. The naive line (pay a creature's mana cost reduced by to keep it) was never the point; the point is the first clause read in isolation. For two mana at instant speed, you put a creature onto the battlefield, and if you decline the discounted cost it is sacrificed before the spell finishes resolving. That sacrifice happens during resolution, with no priority pass in between, so the creature never lives long enough for either player to respond to its presence. What makes the card a combo engine is the order of operations afterward: any enters-the-battlefield trigger the creature has, and any dies trigger from the sacrifice, go on the stack once Flash has fully resolved. You have paid to fire off a creature's triggered abilities and then watch the body leave, the death clause irrelevant because the triggers have already been banked.

That is why the card holds a permanent place on banned lists. Point it at a creature whose ETB or death trigger does something irreversible and the price tag becomes the entire setup for a first-turn kill. The exploit cannot be patched at the source: the instant is harmless and the creature is harmless, and the combination is the problem, so the burden has fallen instead on every later creature design that has to anticipate this loop. A clean instant from an era that had no idea what it had made, retired into eternity as a combo enabler with the rate of a cantrip and the ceiling of a kill spell.