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, it's possible to actually walk around E. Gadd's lab. There are two doors, one of which says "Training" that most likely went to the Training room, and one that likely went to the Ghost Portrificationizer room. There is also a functional ladder, but it doesn't lead anywhere. This suggests that in an earlier version of the game the player could travel to E.Gadd's lab by means of a ladder somewhere in the mansion.

In earlier versions of Luigi's Mansion there was a timer on the HUD. According to Nintendo Power Magazine, this was because Luigi had only 24 hours to save Mario. It was never explained what happened if he failed, but fans have speculated that Mario, along with the Mansion, would have disappeared, or that Luigi would have been possessed. This speculation is based on leftover 'Ranking' files in the game's data. Three unused renders show Luigi holding a flower and looking depressed, him looking happy and making a peace sign, and another of him making two peace signs.

Before going into the Training Room in the original game, a menu will appear giving the player different controller options. After waiting for a couple of minutes, the music track will change to a funky beat, 19 notes will play, and the original music will then return. These 19 notes are actually Totaka's Song, a song that is hidden in almost every game composer Kazumi Totaka has worked on. In the game's Nintendo 3DS remake, this control selection screen only appears during the first time you go to the Training Room, giving you only one chance on that save file to find this Easter egg.

On the file select 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 that it reads, "Welcome to our mansion." The copy and erase messages are also spooky: when choosing a file to copy, the message says, "More files means more fear." Selecting erase causes the message to alternate between "You can erase the files" and "but not the fear."

Luigi's Mansion was originally developed 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. Stereoscopic 3D would later be used in the game's Nintendo 3DS remake.