first watch the video:
This is an example of what is called “inattention blindness” or “change blindness”. The idea is that people often miss large changes in their visual field. This has been shown in many experiments.
hope you enjoyed this bit
Read more: http://www.businessinsider.com/100-things-you-should-know-about-people-2010-11#1--you-have-inattention-blindness-1#ixzz1Hk6NQYT9