The training room is completely modeled out in the 3DS version compared to the original release on the GameCube. However, in both cases, the player cannot use the Game Boy Horror, making it impossible to see through normal means.

The image of Luigi's shadow casting into the mansion's entrance seen in the pause menu is actually taken from the game's E3 2001 announcement trailer.

The book that the ghost Neville is seen reading is titled Mario Story, a reference to the Japanese title of Paper Mario.

The Hidden Mansion mode in the European version was heavily altered and is much harder to complete compared to the Japanese and North American versions:
• Everything is mirrored from left to right.
• The mansion is much darker compared to the main game.
• Some of the ghosts that normally appear later in the game appear earlier.
• Boos have more health and are more difficult to be sucked up by the Poltergust.
• Chauncey's rocking horses are harder to avoid, since they are larger, zig-zag towards Luigi, and travel faster than in the normal mansion.
• In the Bogmire battle, there is time to capture him before he disappears.
• During the Boolossus battle, there is a small cutscene where Luigi must hop on the Poltergust and ride around on it while trying to pop him, making Luigi harder to control.
• In the King Boo & Bowser battle, Bowser's bombs have a shorter fuse.

Some Japanese text can be found in the game's files which translates to: "And... a present from the doctor!"

This is most likley placeholder text for when Prof. E Gadd gives Luigi the Poltergust.

During development, the nursery was originally called the "Child's Room", and Chauncey was missing. Instead, the room was filled with ghosts. A stuffed rabbit and two teddy bears were to the far right on a shelf and the different styled crib and mat were further away from the wall. After beating the room, coins and a normal key would appear out of a treasure chest, as for what door uses that key is unknown. It's unlikely it would have been for the Area Two door, but possibly not, as the door was accessible from the start.

The pitchforks carried by the Ghost Shy Guys are the same pitchforks carried by Beezos in Super Mario Bros. 2.

The world record for beating Luigi's Mansion stands at 9 minutes and 43 seconds. This is due to a series of glitches that allow the player to get out of bounds after the first boss fight and head straight to the final boss room.

Using the Poltergust on a Ghost Shy Guy will cause his mask to fall off, revealing his face and making him vulnerable. This is the only time in the entire Mario series that Shy Guy's face has been shown.

E. Gadd's shack is not outside of the mansion in the demo which was shown at E3 2001.

The tube connected to Luigi's vacuum was animated in the demo at E3 2001 to show the ghost expanding the tube. This animation was not included with the redesigned Poltergust 3000 seen in the final game.

In a prerelease build of the game, the music for E. Gadd's Lab was its own original song, rather than the techno remix of the main theme used in the final game.

There is a model in the game of a tall, faceless, twisting creature known only as "Elh". It appears to have been created a month after Luigi's Mansion was shown at E3. It has no textures, but it has some animations relating to fire and water. It's unknown what this creature would have done or why it was removed.

In a prerelease build, there was a chef ghost inside the kitchen who would throw tomatoes at Luigi. The model of the tomato is still in the game's data, and seems to be one of the oldest models in the game.

Within Luigi's animations appears to be a variation of the same animation used for when Luigi is being held by the Grabber Ghosts. This choking animation is internally called 'poisoned' and he slaps himself when he stops choking. It is quite possible this was the original animation used when he came in contact with a poisonous mushroom.

There are two ghost characters in Luigi's Mansion called Henry and Orville, who each use a car and plane in their battles, respectively. The first ghost is a reference to Henry Ford, who was a major contributor to the automobile industry, while the second is a reference to Orville Wright, who, along with his brother, designed and flew the first powered, fully-controllable fixed-wing airplane.

An early, unused, model of Bogmire can be found in the game's files. This early model shows Bogmire with a slightly larger mouth than that of the final version.

The first 8 notes of "Toad's Theme" from Luigi's Mansion are the same as the first 8 notes of "Toad's Message" from Super Mario 64. The songs are each played while talking to Toad in their respective games.

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, 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.

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 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 Luigi confronts King Boo before capturing all of the portrait ghosts, 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.