I was also very impressed by the Simpsons episode "Angry Dad: The Movie" in which the family goes to a bunch of awards ceremonies because Bart's short film is winning awards.
The Simpsons animation team did very credible clips from short films in the style of Pixar, Aardman Studios, and a few nods to other animation styles that I've seen before but don't know the name of.
It was cute when Lisa shook hands with Nick Park, his fingers came off, he stuck them back on and commented "No problem. I'm more clay than man now."