Black Swan Event
A metaphor that describes an event in history that comes as a surprise, has a major effect, and is normalized after the fact.
Named for an ancient saying which presumed black swans did not exist, but was later modified to mean the opposite when they were discovered in Australia. Examples of Black Swan Events include wars, recessions, and disasters. The metaphor was later developed by Nassim Nicholas Taleb.
As I update this in 2020, I can't help but be confronted with the COVID-19 pandemic as the ultimate Black Swan Event. But there are others that affect our everyday lives, such as a windfall of winning a lottery, to the untimely death of a loved loved one. Hindsight is always 20/20, and people have a tendency to heap guilt upon themselves for the personal Black Swan Events of their lives, but we must also recognize that their existence is by definition nearly unpredictable.