Things that lie on the boundaries

Things that are outside the normal often are not paid attention to. Many people even ignore whatever does not match the regular pattern of things. Thus when an outlier has a massive effect on the system, people are caught by surprise.

Also, many people tend to like certainty. However the nature of this world seems to be notoriously difficult to predict.

When a totally unexpected thing does occur, it comes to be known as a Black Swan.