When the observer is focusing attention on one thing and changes occurs simultaneously with a visual disruption, the observer can fail to notice large changes to objects or scenes. The idea is that people often miss large changes in their visual fields.
This shows that people might have seen “it” but that doesn't mean they actually paid attention to “it” or consciously interpret it.
Monkey Business Illusion
Diapositiva 3
Don’t assume that people will see something on a computer screen just because it’s there. This is especially true when you refresh a screen and make one change on it. Users may not even realize they are even looking at a different screen.
If you want to be sure that people notice a change in their visual fields, add additional visual cues (such as blinking) or auditory cues (such as a beep).