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

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.

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.

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.

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

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.

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 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.