Fans of Pixar know to look for certain hidden items, or Easter Eggs, sprinkled throughout their films. For instance, John Ratzenberger, who voices a character in every film, played Scottish guard Gordon in Brave. Those with great vision would have spotted two other tributes hidden in the same scene.
The Pizza Planet Truck is seen in the still below on the Witch’s work bench.
And Pixar usually places something from their next film to be released. In this case it was Sulley, the lovable blue monster from Monsters University, carved into a block of wood, also in the witch’s cabin.
Sulley is the only one I saw during the first viewing of the movie, I spotted the Pizza Planet Truck on the second viewing. The one tribute I haven’t seen yet is A113.
A113 is the number of a classroom at CalArts that many Pixar artists attended (John Lasseter and Brad Bird to name a few) and it finds its way into many Pixar films. My understanding is they are written in Roman Numerals somewhere. Look for ACXIII next time you see the film.