Pixar films, in working with Disney have taken the approach of putting easter eggs into their films. Easter Eggs are hidden references from other films or animations that if the viewer is not paying attention will miss. Disney has done this with many films in the past such as The Hunchback of Notre Dame, where you can see Belle in the crowded streets of Paris from Beauty and the Beast. THe people at Disney/Pixar put not only refrences to other films but they also use a secret code A113, often seen as A-113 or A1-13. This is Bard Bird’s way of paying homage to CalArts, his old school. Bird is an animator and screen writer for Disney/Pixar and A113 is the old graphic arts studio where him and his friends would work. HERE is a list of locations you can see the A113 refrence including from Bird’s episodes of Family Guy and The Simpsons.
Did you catch it? Code A113 inserted obviously in Wall-E. Sometimes animators are more subtle and sometimes the audience can even miss the reference if it is part of the main story arc. Many other film makers such as George Lucas and Stephen King also have numbers of importance that show up in many of their works so the idea is spread over many genres. For easier references but often harder to spot moments a person can watch any Pixar movie to catch reference to other films especially Toy Story. Many people do not realize that Pixar references movies before they even release them. Easter Eggs of Wall-e can be found in older films such as Toy Story and Nemo can be found in movies such as Monsters Inc. which came out years before. These can be seen clearly in the following clip:
So next time your watching a Pixar or Disney film, pay extra close attention because you may be seeing characters from movie that havn’t even begun production yet.
ETA: Responded to Martin, Finch, and McCarthy