The animated films of Pixar are all rich with details that reward multiple viewings, and for years fans have exchanged the small clues online that suggest that all of the movies are somehow linked. Taking it further, Cracked put out this video a couple years ago discussing the theory that all Pixar movies take place in the same universe, and taken together they spell out "a timeline for the robot apocalypse." Hobbyist film critic Jon Negroni took up that theory and traced his own visual examples of Pixar's little Easter eggs on Tumblr and on his blog, and in brief it goes something like this: If Brave takes place, chronologically, first on the timeline, one can trace the witch's magic in that film to the sentient toys, animals, and monsters in films like Toy Story, Finding Nemo, and Monsters Inc. Pixar has also made references to the same evil corporation multiple times, BNL or Buy-N-Large, which is the global enterprise that rules everything in the time of Wall-E, but is also the company that's trying to oust the old guy from his house in Up, and makes appearances in Finding Nemo and Toy Story 3 (Buzz Lightyear's batteries are BNL brand, as we see in one shot).
You can, in fact, go kind of deep with this. Sully the monster from the Monsters movies appears as a carved figure in Brave, too, and there's also the theory that the witch in that movie is actually just the grown up little girl from Monsters Inc., Boo, who time-travels back to medieval Scotland or whatever to find Sully, and her love for him is the crux of the entire animated universe. Also, the young girl from Inside Out showed up in Finding Dory, did you catch that?
There's also a piece of this about the AI machines that originate with the Omnidroid in The Incredibles, and evolve into talking cars (Cars and Cars 2), and later Wall-E. Also, A Bug's Life could be depicting Earth post-apocalypse, after Wall-E helps that plant grow, but without many humans around.
And there's a Pizza Planet delivery car that shows up in multiple movies.
Today Pixar provides a three-minute video, posted to the Toy Story Facebook page, that traces some (but not all) of these Easter egg connections, though it doesn't confirm the full theory, because the Pixar people were probably too busy making movies to come up with this overarching, multi-century-spanning, totally insane plot structure.
Below, fodder for more stoner conversation.