There is actually a great example of this shown in pictures in the "How to pattern your shotgun" thread in the "Tips and Tactics" section. SwampCollie posted a picture of just a turkey head target that had been taped to a big sheet of paper and then he posted a photo of the whole sheet of paper it had been taped on. If you had just been shooting at that small turkey head target you would never have know that center of that pattern actually missed it. You would have simply thought "gee that shell didn't pattern very well". When the truth could be (as SwampCollie demonstrates) that it threw a pretty good pattern just to the left or right from the point of aim. And trust me many shotguns "DO NOT" throw the center of their patterns directly to point of aim and require an adjustable sight to get POI and POA in line with one another. And that goes for the cheapest shotgun out there all the way up the much coveted Benelli's.