a contingency break; inattentional blindness
This scene is an example of a contingency break. A contingency break is when, in a piece of media (usually children movies or TV shows) a scene occurs that is immediately retconned in the next scene. A common example of this is in children's cartoons, when a character may have gotten their clothes dirty in one scene, but they are back to normal in the next with no time for them to have been cleaned. This applies to the movie <em>Shrek</em>, as the three blind mice are turned into horses in one frame, but are back to the status quo in the next.
Inattentional blindness is the failure to notice a fully visible, but unexpected, object/action because one's attention was on another object/action. A contingency break can be considered a "real-life" example of inattentional blindness because, if this scene occurred in real-life, you would not notice the mice turning back to normal as your attention was not focused on them.