In Luigi’s Mansion 3, after defeating Amadeus Wolfgeist, the cutscene for obtaining the elevator button to the 6th floor can sometimes be obscured by heavy smoke that seems to appear instantaneously upon collecting the button. This is actually the smoke used for the effect of breaking the piano during the battle; however, a glitch causes it to reappear during the cutscene despite the battle being over.

_{Main Blog | Twitter | Patreon | Source: Luigi’s Mansion 3 (Switch)}

In Luigi’s Mansion 3, using the Suction Shot to toss an object over Luigi’s head while another object is between it and Luigi will result in the intervening object pushing Luigi in the direction of the toss at high speed.
_{Main Blog | Twitter | Patreon | Source: myself, Luigi’s Mansion 3 (Switch) on original hardware}

King MacFrights, the boss of the sixth floor in Luigi’s Mansion 3, has a glitch in his programming that causes Luigi to rapidly snap to the end of his lance when performing his charge attack if he runs over Gooigi in the process. Note that King MacFrights always targets Luigi and two-player mode is needed to move Gooigi into his path for this to occur.

This is likely due to some portion of the code treating Gooigi being hit as Luigi being hit, and Luigi’s model being transported to the lance as a result.
_{Main Blog | Twitter | Patreon | Small Findings | Source}

During the brief segment where E. Gadd follows Luigi near the beginning of Luigi’s Mansion 3, it is possible to see E. Gadd’s eyes by using the Strobulb on him. In his ensuing animations, his glasses are temporarily raised, allowing his eyes to be seen. The best place to do this is during the elevator ride, as E. Gadd can be brought closer to the camera than usual.
_{Main Blog | Twitter | Patreon | Small Findings | Source: myself, Luigi’s Mansion 3 on original hardware}

In Luigi’s Mansion 3, the physics inside the Inverted Bedroom on the 11th floor are unique due to the room’s gravity gimmick. This causes some objects in the room to randomly oscillate wildly before settling in their default place, like this lampshade.
_{Main Blog | Twitter | Patreon | Small Findings | Source: twitter.com user “CripplePickle”}

In Luigi’s Mansion 3, the AI used for characters following Luigi sometimes causes them to turn around in circles if Luigi takes a certain path. Immediately after rescuing E. Gadd in the beginning of the game, descending the stairs on the right side with Luigi in the manner depicted will result in E. Gadd running in a circle indefinitely until Luigi approaches him again.
_{Main Blog | Twitter | Patreon | Small Findings | Source: twitter.com user “achristrophe”}

In Luigi’s Mansion 3, a map of the Last Resort hotel can be found near elevators and examined. Comparing the map to the way the hotel actually appears in-game, it becomes apparent that the two do not fully correspond to each other.

The in-game hotel has fewer visible floors than the map, with some being absent, like the Paranormal Productions floor (right above the greenhouse floor on the map, decorated with a studio light). Some visible fixtures are in different places; note that the dome on the right side of the hotel is near the top of the map, but near the middle of the in-game hotel.

In all, the map is much more accurate to the layout of the inside of the hotel, which suggests that the design for the outside of the hotel was created much earlier and simply not updated as the floor layout changed during development.
_{Main Blog | Twitter | Patreon | Store | Source: 1, 2: myself, Luigi’s Mansion 3 (Switch) on original hardware}