Pixar does that a lot, where they put little "insider joke" kind of stuff in each movie. The bouncing ball (from the Pixar short film with the hopping lamps) shows up in Toy Story 2, I think. Woody shows up in A Bug's Life Outtakes.
Yeah, I didn't really like A Bug's Life, but the outtakes were quality. Well, moreso than the film, at any rate.
I guess that's one cool thing about drawing (or rendering) your movies: you can do stuff like that a lot easier.