Disney/Pixar's Monsters University features a few such gems, three of which are revealed here:
A113 - The School of Scaring classroom number is A113, which refers to the former classroom of John Lasseter, Brad Bird, Pete Docter and Andrew Stanton at CalArts. The number makes an appearance in every Pixar feature film.
Pizza Planet - The Pizza Planet truck is in the driveway of the JOX fraternity house when Mike is riding Archie the Scare Pig down Greek Row. The Pizza Planet truck, which first made an appearance in "Toy Story," has made a cameo in nearly every Pixar feature film.
Luxo - The Pixar Luxo ball is in graffiti on the wall behind Claire Wheeler and Brock Pearson during their introduction to the Scare Games' first challenge. Pixar Animation Studio's groundbreaking short film "Luxo Jr." was the first computer animated short film released by Pixar during its first year as a studio in 1986.
Go behind the scenes and learn how the animation for this film was done in the CGW July/August 2013 issue.