The number A113 is hidden in almost all Pixar movies. Many of the animators at Pixar are alumni of CalArts, the animation school founded by Walt Disney, and A113 is the classroom where character animation is taught.
Here are a few places it's hidden:
TOY STORY: A113 is license plate number on Andy's mom's van.
WALL-E: A113 is the directive given to the Axiom that says never to return to Earth
UP: A113 is the courtroom that Carl goes to after he hits the man with his cane
THE INCREDIBLES: A113 is a room in Syndrome's lair