Luigi can't jump as normal because he is carrying a cumbersome vacuum around on his back.
Wondering why doesn't King Boo have his Jewel Crown in later games? Because Luigi sold it to buy his new mansion (even though the jewel is worth 1 COIN).
Luigi doesn't get to the mansion until Mario's been captured. Player 1 lost a life, and now it's Player 2's turn.
You learn from a diary entry in the study that one of the ghost twins is susceptible to cold. This is because planes weren't protected from the weather when they were first built by Orville and his brother.
Brentalfloss points out that it's weird that Mario is unable to escape the painting, considering that he could go in and out of paintings in Super Mario 64. This could simply be explained by assuming that the painting Mario was trapped in works differently from the paintings in 64, but even if it did work the same, there's another explanation. In 64, Mario can't exit a painting unless he gets a Star, or he dies. As far as we know, not only are there no Stars in there, but there is nothing that could kill Mario.
This is further supported by the fact that Luigi does find Mario's Star but it's hidden away inside of something (the Moon) rather than in the mansion proper, which suggests that Mario could've had a way out of that painting if he hadn't lost that star.
E. Gadd seems none too shocked that the person making surprise contact with them in the 3DS remake is his own future self. But that makes sense if you remember this isn't the first time he's been the "past self" in time travel shenanigans.
If you look carefully, you'll see that all of the ghosts on the Second Floor are related—Neville and Lydia are married with three children, and Uncle Grimmly, Aunt Petunia, and Nana are their extended family. The only exception is Sue Pea, although many fan theoriesHand Wave this by claiming she's a cousin.
Her name is Miss Petunia. But Nana's diary entry about pranking her makes it seem like they're family, so her being an aunt is possible.
Why does the first child you find in the Twins' Room accuse you of cheating after you uncover the second? Simple—if you're doing it the "right" way, that first twin just saw you expel an element on the box! Of course that's against the rules!
A weird meta example: the game is kicked off by Luigi winning a mansion in a contest he didn't even enter. Or, to put it another way: in true Luigi fashion, he won it by doing absolutely nothing.
Why does King Boo and the rest of the ghost seem so much more terrifying and Knight of Cerebus in this game when compared to thier other appearances. Simple because in those games we are seeing the ghost how the Fearless Mario sees them, but in this game we see how the Phasmophobia Luigi see them.
Why did Luigi fall for such an obvious scam? He didn't. He was there to find Mario. Mario fell for the scam. And "Your brother won a mansion in a contest you didn't know he entered" is a lot less unbelievable.
And then there's the heart quotes. Most of these are harmless little tidbits of speech that the ghost is keeping to themself. But then there's Sue Pea. It's a hint at how to beat her, but think. What if that quote was her last words?
To people who played Paper Mario, Luigi's cowardice seems weird. In that game, he was eager and adventurous. However, if you read his diary at one point, you can see he was already scared of ghosts. And the first game with Luigi's fearful personality? Luigi's Mansion, where he has to travel through a haunted mansion. Luigi is traumatized. And nobody seems to be helping him.
Luigi's humming and calling for his brother make up a chilling, creepy "soundtrack".
There's those two ghost twin children who love to play hide and seek, nothing inherently wrong with that right? Well if you happen to find a diary in the study, reading an entry reveals that they both like to hide during the game, sometimes causing the game to go on for a very long time... meaning two kids probably starved themselves to death over a children's game.
So Word of God is that Chauncey was actually born a ghost; the immediate assumption is that Lydia and Neville somehow had another baby after they died, removing the inherent horror in a ghost baby. But then... Being born a ghost could also mean that he's the result of a miscarriage, a stillbirth, or the result of Lydia dying while she was pregnant.
It's believed by some that King Boo's Bowser "mech" isn't a mech at all, but rather Bowser's corpse, with the Koopa King having been killed by Mario in a previous game.
Not made any better when Madame Clairvoya herself is shocked by Bowser actually being relevant at all in King Boo's scheme, since she points out that Bowser had been soundly defeated by Mario, and specifically theorizes that King Boo has revived Bowser. If Bowser was still alive after his defeat, then said choice of words wouldn't make much sense. This is made even more horrifying when this also presents hints that Mario may have killed Bowser at some point in the past. Luigi's Mansion is a dark game indeed.
So, the portrait ghosts. Not only are they technically real, albeit dead, people being trapped in paintings against their will (and not all of them are even trying to hurt you,) but: Have you ever gone to the gallery and noticed how they don't move inside the paintings like how Mario does in his painting? They just stand still, forever.
All the area 1 portrait ghosts plus Henry and Orville are members of the same family. That means something horrible must have happened to kill all five at the same time. It gets worse if you notice there is also an Uncle Grimley, Aunt Petunia and Nana who could also be related to this family.
Nana has the key to Henry and Orville's room, implying she is their grandmother, but Petunia is never referred to as "Aunt" in the game.