Stale Cupcakes
From Nookipedia, the Animal Crossing wiki
"Stale Cupcakes" (ブルーおにぎり, Blue Onigiri) is a K.K. Slider song. It is a quiet song, when played in the house. It did not appear in the original Animal Crossing. It appeared in Animal Crossing: Wild World when the player put in an invalid song name. It is a secret song, but Julian and Diana (as well as Rodeo in Animal Crossing: New Leaf) have it playing in their houses.