The October 2001 issue of Nintendo Power included a page with preview screenshots of Luigi's Mansion, including a screenshot of an earlier build of the Safari Room with the caption: "When your ghoul-busting mission takes you to the trophy room, proceed with caution. If you meet up with the ghost of a hunter, he'll want to add Luigi to his collection." This sparked rumors among players that the Safari Room originally featured a hunter boss who wanted Luigi's head as one of his trophies, and that it was removed from the game for being too frightening to the target audience of children.

However, no evidence of a hunter ghost is present in the game's data despite the presence of other unused ghosts, and no direct mentions of the boss exist in previews prior to the game's release in Japan in September 2001. The caption was likely an attempt at hyperbole rather than a serious indication that a hunter-themed ghost was in the game, as the article was written after Luigi's Mansion was released in Japan, meaning all characters would have been finalized long before the article was published.

At Spaceworld 2000, Shigeru Miyamoto showed off a prototype for the GameCube controller. The most glaring difference between this controller and the final design are the color and shape of three buttons:
• The A button being blue, like the Z button rather than green, but keeping it's circular shape.
• The B button being green, like the final A button rather than red, and bean shaped like the X and Y buttons rather than circular.
• The start button being red like the final B button as well as being bulbous and protruding as compared to the final start button being flat, grey, and made of rubber.

An accompanying tech demo used assets from Luigi's Mansion including one of the basic ghosts which reacted differently depending on what button was used:
• Using the A, B, X, and Y buttons made it spit out the corresponding letter
• Stretching for the L button and squishing for the R button
• Emitting a slower version of Boo's laugh for the Z button
• Emitting a normal ghost noise for the control stick
• Changing orientation for the C-stick
• Emitting sounds similar to that of Pikmin for any of the D-pad directions

The inclusion of the Pikmin noises makes sense as Luigi's Mansion contained a trailer for Pikmin upon release.

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.