What is the conjunction fallacy?

Rating a detailed story as more likely than a simple one. Every added detail can only make it less likely.

The famous version is the Linda problem: given a vivid description, most people rate "bank teller AND activist" as more likely than "bank teller" alone, which is mathematically impossible. Detail makes stories feel truer while making them less probable. AND is always a discount.

Markets serve this one on a plate: any question of the form "X happens AND by this date AND in this way" must be priced below each piece alone. When a vivid, specific scenario is trading rich because it tells a great story, the boring NO side is quietly on sale.