Black Swan event (noun) 

Black Swan is an unpredictable event that is beyond what is normally expected of a situation and has potentially severe consequences. Black Swan events are characterized by their extreme rarity, their severe impact, and the widespread insistence they were obvious in hindsight. (Source: Investopedia)

As I used it today: “The point is that Black Swan events do happen. And when they create recessions, thousands of businesses go belly-up and millions of workers lose their jobs.”