"Upside down L is real 2401" is a message in the DS version of Super Mario 64 and involves Waluigi in it and him rumored of being in the game, as well as fake cheat codes and hacks, similar to Luigi being in the original game (sometimes Wario and Waluigi), who's now actually playable. Like the N64 version in the back of the castle in the Boo's courtyard, there's an Eternal Star Statue on the fountain saying Upside down L is real 2401. People thought upside down L stood for Waluigi and he could be in it.