I remember seeing this 'count the passes' video a number of years ago and being completely blown away by it. If you have never seen it, check it out before reading any further.
The video is a famous demonstration of what is known in the psychology world as inattentional blindness or what some researchers call selective attention. Inattentional Blindness is the failure to notice some object which is fully visible but unexpected because attention was focused on some other object or task.
There are four criteria which must be met for a failure of awareness to be considered an example of inattentional blindness. 1) The observers fails to notice a visual object or event. 2) The object or event is fully-visible and observers readily see it if they are looking for it. 3) The failure to notice results from engagement of attention on other aspects of the display and not from aspects of the visual stimulus itself. 4) The object or event is unexpected. 1
Four factors which may affect inattentional blindness include:
Conspicuity - An objects tendency to capture our attention.
Mental Workload - How much of our attention is divided unto other things.
Expectation - How expecting certain things can have a tendency to block out other possibilities.
Capacity - An individuals capacity to focus attention which is also related to how much attention is needed for a particular task. 2
Inattentional blindness is often a major topic in public safety research which seeks to limit the use of cell phones while driving. 3
Here are a couple of entertaining videos on inattentional blindness. As explained above, for this phenomenon to work the viewer must not be expecting and thus looking for something peculiar. With this in mind you may not be fooled by the following but then again, maybe one will get passed you.