Reading about Nassim Taleb's Black Swan theory - that we are disposed to think that events are predictable and that we reinforce this with hindsight.
Wikipedia cites an example Taleb uses to explain his theory is the events of 11 September 2001. Namely that
"the attack on the World Trade Center and The Pentagon using commercial airliners was virtually unthinkable at the time. However, with the benefit of hindsight, it has come to be seen as a predictable incident in the context of the changes in terrorist tactics."
I beg to differ. If security analysts had thought in terms of 'what powerful destructive force could a terrorist get his or her hands on?' then an airliner would have been on the list.