Testing of airport screeners, using images of guns and banned objects during the airport X-ray of bags, had some interesting results. When using a library of 250 images, the performance of the screeners (how often they detected the items) went up over time. When the image library was changed, performance dropped dramatically.
The new images were all still the same types of banned items, but they looked different. They were different types of guns, or knives pointing in different directions.