The joke behind mysterious A113 code

The mysterious combination of numbers and letters 'A113' that has been popping up in Pixar films for over a decade is well known to people in the animation industry. The secret behind the code, which made appearances in various forms in the Toy Story trilogy, Finding Nemo, A Bug’s Life, Up, Brave, as well as animated shows like The Simpsons and American Dad, and even live-action movies like Hunger Games: Catching Fire, is that it's an intentional inside joke. [more]
A113 (pronounced A1-13) was the classroom number used by character animation students at the California Institute of the Arts. Many of its alumni (including Pixar staff) have used the number in their professional works.

CalArts illustrious alumni (L to R) Pete Docter, Andrew Stanton and Chief Creative Officer of Pixar and Walt Disney Animation Studios John Lasseter posing outside room A113 in the 1990s