The very first episode of the series where all six of the main ponies are completely absent. Not one of them makes even so much as a cameo.
The "Mane 6" have no spoken lines in this episode.
In the scene where Apple Bloom is trying the life painting class, her easel is set up next to that of a pony who bears a strong resemblance to Andy Warhol, right down to his pop art painting technique. Said pony is also sporting a tomato soup can cutie mark. Two other ponies in the room also reference famous painters - one Picasso, with a cutie mark consisting of some cubes, and another Salvador Dali, painting surrealist ponies and bearing a flamboyant moustache just like Dali's.