We often see what we expect to see and miss other things because we are not looking for them.  A famous experiment illustrates this. A group of people is passing balls around. Observers are asked to count the number of passes made. During the time the group is passing the balls around, a person in a gorilla suit walks into the group, struts around, then moves away.  About half the observers say they did not see a gorilla.  You can see an illustration of this experiment here.