Pixar Self-Reference
Was that Rémy the rat from Ratatouille that you saw in the background of Toy Story 2? Well… probably not. But there are plenty of callbacks and cameos in the collective works of Pixar, from the overt to the barely visible. Jim Hill has quite a comprehensive (or at least very long) investigation of the studio's self-reference.
Hill goes into impressive detail, even citing the appearance of the birds from For the Birds in Cars. Said appearance lasts only a tenth of a second.