Ascended Fanon: The names of the three Hitchhiking Ghosts (Ezra, Phineas and Gus) originally came from cast members and were so popular in the Mansion's fandom that Disney eventually adopted them as official ones.
A story circulated that the Bride's ring was embedded in the exterior exit path of Walt Disney World's Haunted Mansion. It was actually the remains of a crowd-control stanchion that had been cut down. It was removed during 2007's Re-Haunting. In 2011 an "official" Bride's ring was embedded in the path.
The Bus Came Back: There have been reports that if you ask a cast member or staffer where the Hatbox Ghost has been all these years, they'll say, "He was always in the attic, you just couldn't see him, but he could see you!"
The Danza: Madame Leota is named for Leota Toombs, the Imagineer who provides the character's face. Her daughter has taken on the face role for Haunted Mansion Holiday.
She then providied the voice and face for Little Leota.
Executive Meddling: According to one of the engineers, they've wanted to bring back the Hatbox Ghost since at least 1999 (then, they were celebrating the ride's 30th anniversary), but everytime they tried to pitch the idea to Disneyland execs, they were immediately shot down with the reasoning, "the character's effect didn't work before, so why would it now?"
Tony Baxter, and a few other engineers have (discreetly) implied that Constance Hatchaway is a product of Disneyland executives, not the ride designers themselves. One of the Hatbox Ghost creators has stated that now that "Hattie" is back in the ride, his next goal is to try and get the bride back to a form that's closer to her original incarnation.
Fan Nickname: The bride who came before Constance was often called "Emily" by fans.
No Export for You: None of the Mansions are identical. Tokyo never got the Constance update to the Attic, while Disneyland never got the update of the Constance effect that WDW got. Walt Disney World also has the Music Room, Library, and Endless Staircase, scenes not in the Disneyland Mansion. Conversely, Disneyland is the only attraction to have gotten the Hatbox Ghost 2.0.
Permanent Placeholder: Imagineer Leota Ann Thomas (Leota Toombs at the time — not kidding) was originally supposed to be a stand-in for the crystal ball effect. Though Eleanor Audley's voice was used as planned, the Imagineers liked Leota's performance so much that they ended up using it, and the character was given the name Madame Leota. If you're wondering what Leota sounded like, that's her voice asking you to "Hurry back" at the end, (although this was a forced breathy voice). The fans and Disney alike call that ghost "Little Leota" as a result. (Officially, it's "the Ghostess".)
Captain Nemo's organ, from Twenty Thousand Leagues Under the Sea, in the Disneyland version's ballroom. (It is the only organ to be genuine; the WDW and Tokyo versions are non-functional copies of Nemo's one).
The mold for Hitchhiking Ghost Ezra's head was used for pop-up ghosts, portraits in the Corridor of Doors, and the original Hatbox Ghost.
It was originally supposed to open in 1963, but the project's development was delayed by the 1964-65 New York World's Fair, as well as by the death of Walt Disney. The Haunted Mansion didn't open until 1969.
After 46 years, the Imagineers finally had the technology needed to bring the legendary Hatbox Ghost back into the ride.
The Hitchhiking Ghosts were tossed in at the last second.
A park visitor caused a piece of the glass separating the Doombuggies from the scene to crack. However, what would have been fairly expensive and awkward repair job was allowed to remain because the crack just happened to look very much like a spider's web, so a fake spider was added on top of the crack.
The ghosts dancing in the ballroom in the Anaheim version are animatronics reflected through clear glass, which resulted in the ladies leading the dance. Although this would be easy to correct, it was left in as part of the gag.
Updated Re-release: The ride has been redone several times with effects that don't break down nearly as often.
The Walt Disney World and Tokyo Disneyland versions of the ride contain additional scenes not present in the Disneyland original. Phantom Manor at Disneyland Paris takes the loose bits of story present in the original ride many steps further to produce a full history for the house, and also for Big Thunder Mountain, meaning the signature Frontierland roller coaster gets tied to the history of another ride, getting Darker and Edgier in doing so.
What Could Have Been: The Haunted Mansion went through several different ideas. The following concepts are only a very small fraction of what we know the Imagineers considered:
Until pretty late in the game, it was going to be a walkthrough instead of a ride. Some of the creators, such as Rolly Crump, still regret they had to make it a ride to this day (the reasons for the change were not artistic, but practical: it was matter of managing the crowd).
The entrance to Walt Disney World's version of the attraction was originally going to be much more elaborate◊, with guests entering through the front door and using the stretching rooms as elevators (as they do in the Disneyland version). Since there was more space in WDW, there was no need for elevators, since the show building could be on the same level aboveground. As such, in WDW, the ceilings rise instead of the floor descending.
One proposed idea for Disneyland's original ride was to have the trees in the cemetery scene move around, and their branches reach out towards riders. However, the engineers back then couldn't figure out exactly how to get the trees to move like that, and in the years since they've never tried to re-attempt it; so the trees in the cemetery just stand perfectly still. The trees still have faces carved into them, in a manner similar to Snow White's Scary Adventures.
The Haunted Mansion Holiday overlay was planned to be added to Walt Disney World's version of the attraction; but for reasons unknown, they turned it down and the already-built decorations were sent to Tokyo Disneyland for them to do the overlay.
Word of Dante/Word of Saint Paul: The “backstory” (or rather backstories) of the Mansion come from various sources, and it has long been debated in the fandom whether any of these examples is Word of Dante or of Saint Paul:
A lot of the so-called backstory (most notably the Ghost Gallery mythos) comes from Cast Members (that is, the employees costumed and roleplaying as servants who manage the guests during the walkthrough part of the ride).
Inversely, some say that only the current incarnation of the ride is truly canon and what the original Imagineers intended in the 60's is only Word of Saint Paul.
The movies and comics were licensed by Disney legally speaking but never personally approved by the Imagineers themselves (either the old or new ones), resulting in concepts and backstories for them being of shaky canonicity.
Missing Trailer Scene: Early trailers and TV spots had Eddie Murphy saying "Wait until I leave, then make the dark forces come out!" in a very stern tone. In the film, while this line does come up, he says it much differently, in a more panicked, frightened tone.
Dummied Out: Several the Ballroom dancers both have lines recorded, but they cannot be interacted with.
What Could Have Been: There was going to be a version released on the Game Boy Advance, but despite being complete, it was blocked from released by Take 2.
Unused audio files within the game reveal that Leota would be able to cast spells that would cause Zeke to cheat death or stay invincible for a period of time. Another line suggests that Zeke would have to escape from the Reaper after being scared to death (ie lose all HP) to reclaim his body.