“Many people think that it’s a hint that Luigi is in the game and it has something to do with a special code. The real answer is that the programmers put it in there as a joke. They thought people would try and try to figure out what it means. It doesn’t have any meaning at all.” ―Michael D. Chandler, Nintendo of America, Inc.
"L is Real 2401" is an Easter egg/theory in Super Mario 64. It involves the character Luigi and his absence from the game. This has led to many theories and fake cheat codes about his nonexistent appearance.
Super Mario 64 was originally going to be a 2-player game, with Mario and Luigi as playable characters. But the system's capabilities at the time prevented that from happening, and Luigi was taken out of the game. Luigi did, however, finally make his Nintendo 64 debut in Mario Kart 64
In the Castle Courtyard where the Cats later reside, there is a statue in the heart of the courtyard. The statue depicts a giant star with a blurry inscription beneath it. Although it is hard to read can, the text on the inscription can be interpreted as either "Eternal Star" or "L is real 2401", depending on the player's viewing angle. While "Eternal Star" made much more sense considering the star directly above the inscription, people quickly latched onto the "L is real" idea. People thought that L stood for Luigi, and therefore Luigi would have been in the game. One theory was that 2401 represented every gold coin in each world of the game, and that by collecting them all, the player would have to come back to the statue where Luigi would then emerge as a playable character.
While there were many theories as to what the inscription could mean besides the one above, the most popular and logical theory is that it was a reference to the game Paper Mario. The game was released in America on February 5, 2001 (02/05/01), very close to the date written on the inscription (02/04/01), and Luigi was a character in that game. There is unintelligible text under the message that might say "In paper M"- although at that time, Paper Mario was known as "Super Mario RPG 2", which might debunk this theory. This could be a reference to the graphical style of the game as opposed to Super Mario RPG: Legend of the Seven Stars
IGN responded to the number of questions about Luigi by offering a $100 bounty to prove that Luigi was not in the game.
- The message was also found in Dodongo's Cavern in The Legend of Zelda: Ocarina of Time.
- The first Mario title for the Nintendo 64, Super Mario 64, didn't have Luigi anywhere in the game. The first Mario title for the Nintendo GameCube, Luigi's Mansion, didn't have Mario show up in the flesh until the end of the game and had Luigi as the only playable character.
- It wasn't until eight years later in 2004 that Luigi would be playable in the DS remake of Super Mario 64 (along with Wario and the previously unplayable character Yoshi) and Waluigi was rumored to appear in it, similar to Luigi in this game.
- In 1, Luigi was actually going to appear in Super Mario 64 2, but was cancelled, due to the commercial failure of the Nintendo 64 DD (Disk Drive).
- On December 2, 2016, in a sub Reddit /r/retrogaming, the theory was debunked way before anyone realized in a letter from Nintendo dated May 7, 1998, which is stated that the "L is Real" message is a joke put in there by the programmers. You can read it here: https://www.reddit.com/r/retrogaming/comments/5g6nf0/letter_from_nintendo_about_luigi_in_sm64_1998/?st=j8zfwn9j&sh=fbd2967f