Black Swan effect:
A black swan is an outlier, an event that lies beyond the realm of normal expectations. Most people expect all swans to be white because that's what their experience tells them; a black swan is by definition a surprise.
Our minds are designed to retain, for efficient storage, past information that fits into a compressed narrative. This distortion, called the hindsight bias, prevents us from adequately learning from the past.
from LEARNING TO EXPECT THE UNEXPECTED, By Nassim Nicholas Taleb,
www.edge.org/3rd_culture/taleb04/taleb_indexx.html