Black Swan Event
A rare, unpredictable event that carries an outsized impact and is rationalized after the fact as having been foreseeable.
Origin
The 2nd-century Roman poet Juvenal wrote of "a bird as rare upon the earth as a black swan" (rara avis in terris nigroque simillima cygno), and for roughly 1,500 years the phrase stood for the impossible. That changed in 1697 when Dutch explorer Willem de Vlamingh encountered actual black swans in Western Australia. Nassim Nicholas Taleb recast the metaphor in his 2007 book The Black Swan.
Everyday Use
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.