You can find some great demonstrations of “flicker” change blindness at http://www.gocognitive.net/demo/change-blindness
The demo allows you to adjust the duration of time for each image as well as the duration of the mask screen in between. Longer pauses will make detecting the change more difficult, while setting the delay to 0ms will reveal the answer. These provide a good example of just how narrow our focus of attention can be, and until we happen to focus on the right area, it can be quite difficult to notice the change.
