Hindsight bias

Hindsight is 20/20. Hindsight bias is the effect whereby people think that past events were predictable, or at least more predictable than they actually were. This is because after an event, the probability of it happening is, naturally, 100%. The bias arises because people ignore the things that didn't happen or the things that didn't cause the event — known as the "availability heuristic". This allows people to point to specific causes of an event (such as a catastrophe) and ask, "Why wasn't something done about it?"

Examples
Numerous cases of hindsight bias can illustrate the phenomenon, but the in 1986 and the 9/11 attacks of 2001 provide well-known examples:


 * In the case of the space shuttle, investigators attributed the failure of the vehicle to a faulty rubber "O-ring" on one of the shuttle's solid-fuel boosters. After the tragedy, inquiry found that this fault was reported as a potential problem, but ignored. Those affected by hindsight bias view this as practically criminal, as the fault was known long before the event, but they completely ignore the hundreds, if not thousands, of other reports that had the same probability of causing the destruction of the shuttle.
 * In relation to the attacks on the World Trade Center, several reports of "something big" were milling around intelligence agencies; however, prior to the attack itself there were many reports of "something big", many of which led to nothing.

Any assessment of how people react to information or intelligence regarding a potential event should be done without the benefit of the knowledge that the event actually happened.

Hindsight bias with events such as these makes it easy to formulate conspiracy theories by claiming that because a government "knew in advance" of an attack or similar, "they" let it happen. In reality, "they" didn't really "know in advance" at all, as the probability of something happening was low.