Contributed by game4brains
There's an unused model of Mario in the game's data. Mario is stretched to Luigi's proportions and is equipped with the back straps for the Poltergust 3000. The geometry of the model is simpler than Mario's final in-game model, containing fewer polygons. Although its true purpose is unknown, it could've been from an unused multiplayer mechanic requiring both Mario and Luigi. The model was stretched out, presumably to fit Luigi's skeleton so that Luigi's animations could be used on the model. The model also uses Luigi's gloves, presumably to make his animations handling the Poltergust 3000 more accurate.
There's evidence suggesting that Luigi's Mansion was going to be an N64 game. On the game's disc, there's a sound effect for getting a star from Super Mario 64 and a cropped image of Daisy's artwork from Mario Tennis. Hideki Kono, the director for Luigi's Mansion, also stated that the game was originally going to be released on the Nintendo 64.
When ghosts are turning back into paintings at the end of an area, a picture of three ghosts playing cards is shown on a screen on the machine. These ghosts are actually from a one second clip for Luigi's Mansion from the GameCube reel at Spaceworld 2000.
Using an Action Replay code, you can actually walk around E. Gadd's lab. There are 2 doors: one of which says "Training" that most likely went to the Training room, and one that likely went to the Ghost Portifacationer room. There is also a ladder you can climb, but it leads to nowhere. This hints that in an earlier version of the game you could actually walk around the mansion and go down to E.Gadd's lab.
Contributed by Zelda
In the earlier versions of Luigi's Mansion there was a timer on the HUD. According to Nintendo Power Magazine, this was because you had to save Mario within 24 hours. It was never explained what happened if you didn't save Mario in time, but fans have speculated that Mario, along with the Mansion, would have disappeared. Or possibly that Luigi would have been possessed. This speculation is based on leftover 'Ranking' files in the game's data.
Before going into the training room, a menu will appear giving you different controller options. If you wait for a couple of seconds, the music track will change to a funkier beat and 19 notes will play during a portion of the song. The track will then change to it's original music. These 19 notes are actually Totaka's Song, a song that is hidden in almost every game composer Kazumi Totaka has worked on.
Contributed by awesomenommer777
If Luigi is talking to Toad or Professor E. Gadd in the Telephone room and lightning strikes, Luigi's shadow appears suspended in the air, as if being hung by a noose. It's not known if this is a graphical error or an Easter egg.
Contributed by Jonahtron
On the title screen, a message will appear and disappear that says, "Welcome to your mansion." However, the "y" in your fades away a split second before the rest of the message, so it reads, "Welcome to our mansion." Also, when you copy a file, the message says, "More files means more fear." And when you erase a file, it says, "You can erase the files" followed by, "but not the fear."
Luigi's Mansion was built with the intention of being in 3D. All Nintendo Gamecube systems support display of Stereoscopic 3D. Nintendo decided to shift focus away from 3D at the time as a 3D screen add-on would have cost more than the Gamecube system itself.
If you confront King Boo before the end of the game, he will say "I will not give up my favorite decoration, I like Mario just where he is." This is a reference to Star Wars Episode VI: Return of the Jedi. When Han Solo is trapped in carbonite, Jabba the Hutt says: "I will not give up my favorite decoration. I like captain Solo where he is." Both Mario and Solo being sealed away seems to reinforce this connection.