Did you know that Pixar, Disney, and plenty of other animation studios have a secret in common? If not, don't worry — the A113 is often hidden in places that are super hard to find. But with the technology and the Internet combined, the code has been cracked for all of us. The Huffington Post has a more detailed write up and a full gallery with most of the examples, but the basic gist can be summed up in a comment on Imgur.
"A113 refers to a classroom number at the California Institute of Arts. It was the classroom for first year graphic design and character animation, where many of the animators at Pixar and Disney, and several other studios, discovered and mastered their craft. The use of A113 in their films is a friendly nod to one another that they once shared a classroom without which they would never be doing what they're doing now."
If you've been following this blog, you should be well aware of how much talent comes out of The California Institute of the Arts. But despite not being surprising, it is still nice to know that this theory, unlike others (the Pixar theory for example), is completely true.