A gorilla went by...

Knowledge@wharton has an entry about the difference between vigilant and operational leaders. It is descently written, but the following example says it all:

In one classic experiment, people failed to notice a gorilla walking through a basketball court because they were instructed to count how often the ball was being passed. The simple task of counting bounces and passes restricts their vision so much that they don't see the gorilla at all. This is what happened to Coke and Pepsi: They missed the gorilla amid the technical, legal and PR problems of the pesticide charge."