The Grand Ballroom scene is the Haunted Mansion's masterpiece, a jawdroppin', show-stoppin', eye-poppin',
Actually, dancing etiquette was the least of the problems they had with the dancers. They almost had a disaster with this effect, but that bit of trivia has lain quietly hidden. Sometimes something is "long forgotten" because someone swept it under the rug and hoped that it would be forgotten.
The effect itself is simple, just a bunch of mannequins spinning on turntables (perhaps too simple—Rolly Crump has been known to criticize it as too obvious). There are six couples in all, two sets of three on two adjacent turntables. The blueprints tell the story.
They store junk anywhere and everywhere inside the HM's. Hey, if the guests can't see it, it doesn't exist.
Strangely enough, in some early photography, there are only three couples, not six. That's what you see in this pre-opening publicity photo, probably taken in the summer of 1969:
It's even on film. Filming the interior of the HM has always been difficult, so the detailed ride-thru footage done by WDI (then WED) for their own in-house use quickly became available in edited form as stock interior footage for the HM. It's endlessly recycled, and you still see snippets of it any time they want to show something inside the Mansions. The famous Disneyland Showtime TV episode featuring the Osmond brothers (aired March 22, 1970) made heavy use of the WED footage. The main point of interest here is that this film was shot well before the HM opened, once again, probably during the summer of 1969. And in the ballroom scene, there are only three couples swirling around ("You can see right through the dancers!" says Donny. Yep, and it doesn't take long, since there aren't very many to see through.)
Some party. Looks kinda lonely out there.
How come three couples, not six? Because they thought they could get by with a single turntable with three revolving couples, multiplied with mirrors. Again, the blueprints tell the tale:
Well, why not? Multiplying objects with artfully arranged mirrors was by now an old Imagineering trick. They had used it to create a swirling mass of ghostly spinning wheels in the original Sleeping Beauty Diorama walk-thru (and in the 2008 remake of this attraction, this scene was restored):
More recently, they had used mirrors to create a vast sea of water molecules out of just a few models, over at the Adventure Thru Inner Space:
So what could go wrong? With a V-shaped mirror, you quadruple the object within the V. See, like this:
Yes I know that the figures aren't reflecting themselves accurately in the mirrors. This a rough sketch, just to give you an idea. It actually looks pretty good. I set up three chessmen on a phonograph turntable and grabbed a couple of mirrors, and it wasn't hard at all to visualize twelve couples gettin' down to that funky sound.
Alrighty, so what's the problem? Well, the scene above is the view from the doombuggy when it's directly in front of (and above) the dancers. But of course the dancers are visible the whole time you're scooting along the balcony. We need to start with more of a side view, like this:
. Whoops, wait a sec. We forgot to put up the mirrors.
. Oh snap.
You know, I sure hope they figured this out before they ordered those huge, expensive mirrors. There is no sign of the mirrors in the publicity shot or the film footage we looked at above, but I suspect that they knew by then that they had a little problem here and a second turntable with another three couples was in the works. Maybe it was not yet built or not yet installed. The thing is, they were still counting on the mirror effect in April. That's when the blueprint above was done. That means that there was a maximum of four months (and possibly much less than that) for them to (1) notice the problem, (2) kick themselves for being so stupid, (3) come up with a solution, and (4) build and install it.
By comparison, Blaine's little "screw-up" with the ladies leading the men was small potatoes.