In change blindness some part of a scene is changed and the change is not noticed by the observer. This can happen when the change is not happening on the retina in a stable condition. It can happen when there is a mask (a blank screen to fast to see), a blink, an eye movement, a change in point of view, an interruption in a action and so on, anything disrupts the continuity of the retina image.
There was some questions of whether change blindness could happen with objects that were at the center of attention. Surely, if you were engaged in a conversation with someone, they could not be replaced with another person without the change being noticed. But they can as Simons and Levin showed in their paper (see citation).
What does this say about our memories? Simon and Levin say:
If we constantly noticed such changes, they would likely detract from our ability to focus on other, more important aspects of our visual world. Change detection as a method relies on the tendency of our visual system to assume an unchanging world. The fact that we do not expect one person to be replaced by another during an interaction may contribute to our inability to detect such changes. … Taken together, these experiments show that even substantial changes to the objects with which we are directly interacting will often go unnoticed. Our visual system does not automatically compare the features of a visual scene from one instant to the next in order to form a continuous representation; we do not form a detailed visual representation of our world. Instead, our abstract expectations about a situation allow us to focus on a small subset of the available information that we can use to check for consistency from one instant to the next.
In effect we delude ourselves as to the completeness of our immediate memory. We remember the unexpected if we notice but there is no guarantee that we will notice.
Simons D.J., & Levin D.T. (1998). Failure to detect changes to people during a real-world interaction Psychonomic Bulletin and Review, 5, 644-649