The black swan theory is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight. The theory was developed by Nassim Nicholas Taleb.

A Japanese stockbroker immediately regrets his decision
Japan avoided the biggest of all „fat finger“ meltdowns on Wednesday when an unidentified large stockbroker managed to cancel rogue trades worth more than £380bn. […]