I'm not sure where it originated, but Cracked.com and Jon Negroni have both suggested it, as well as Slate, based directly on Jon Negroni's post. In short, it speculates all Pixar movies actually exist within the same universe.
The theory revolves around an overall narrative involving the evolution of animals and artificial intelligence. The animals become more and more intelligent (the fish in Finding Nemo, the dogs in Up, Ratatouille) and eventually evolve into the monster in Monsters Inc., but only after a conflict between humans and sentient machines (the toys in Toy Story, Cars (+ Planes) has come to a conclusion (Wall-E). The doors in Monsters Inc are actually doors into the past; Boo uses the same mechanism to travel back in time where she becomes the witch in Brave and sets the whole thing in motion.
This theory covers every Pixar production since Toy Story. That includes:
- A Bug’s Life
- Toy Story 2
- Monsters Inc.
- Finding Nemo
- The Incredibles
- Toy Story 3
- Cars 2
- Monsters University
Some interesting pieces of evidence:
The witch from Brave is actually Boo from Monster's Inc, explaining why she has a picture of Sulley in her shop.
You remember Merida opening doors and the witch constantly disappearing? It’s because those doors are made the same way from Monsters Inc. They transport across time and that is why Merida couldn’t find the witch later in the movie.
Whether intentional or not, this is definitely an fascinating theory and Jon Negroni's take is definitely worth a read.
I'm just wondering if there's any more evidence anyone has unearthed or whether Pixar has publicly commented on this.