There is a 128 page handout with full color pages so it prints horribly. It contains a long rambling history of the locality, with multiple generations of unmemorable characters who have ruled for generations. Make sure you spot the clue on page 37!

I expect there would be a novella's worth of flavor text about Goblin Boss Gloob's early life and how he came to serve Orc Boss Gark. So that when the climactic battle finally comes, we can see the fruition of this tale as Gloob stealthily fucks off while the PCs carve away Gark's 12hp.

Id suggest moving even further into the depths of meta-contrition: perverse layout decisions, pictures with text encroaching their borders, broken links, nested indicies scattered throughout the text, random font switch 60% through, all printed/laid out horizontally in tabloid/broadhsheet format

What about an index? Misspeled of course, and sourced from an earlier draft so that only 25% of the page numbers actually align with the topics? Sorted by page number so looking anything up involves starting at the top and working your way down.

There's a beautiful, full-color, high rez map of the hidden gardens, crystal trees, grottos, glimmering pools, with twist and turns- but its really just one long hallway stacked on top of itself like an intestinal diagram. Nothing out side of the "rooms" (which are just circular areas) are labeled because you can't really do anything with them (e.g. the colored pools are just that).

Print the whole thing in red ink on gray paper, in Fraktur font.

Fraternizing tone, something like "You surely know, oh gentle reader..."
Old memes or slang in writing.
introduce new, 'cool' words to address the readers or the process of play ("Playzzers" for simplest example); rename DM into something long and difficult to pronounce (Galactic Viceroy of Research Excellence, for the quick example) and never use any abbreviation, only the full title, in the text.

Orcs and goblins? No, this castle has Skri'therzm and Mocron-Reiru, which are a race of brutish, chaotic humanoids and race of shorter, brutish, chaotic humanoids. And the skeletons are called "darkbone haunters".

we're going to need room keys after the read aloud text, right? Each room in the dungeon should take up four to five pages in the book. The room keys need to be hyper detailed to the point that they become useless as keys.

Also, why not key 25% of the rooms in pseudo-hexadecimal.

Room 9A2E. Mocron-Reiru living quarters.
7 beds.
→ bed 1. Not made.
→ → old blanket.
→ → → red, made of torn cotton.
→ → → → tear in left hand corner.
→ bed 2. Made, not well though, as if the goblin-reiru was in a hurry to make it. Probably had about 2 minutes to make the bed, with a deviation allowance of 45 seconds.
→ → blanket of blue, cotton. Pillow is straw stuffed into a bag.
→ → → straw bag was made 6 months ago by the farmer Gusa'gel'del'tonathon. The More-Reigru stole the bag from the farmer while he was milking a cow named [cow #1, page xx]
→ Bed 6. Not a bed, actually a chest.
→ → made of water logged wood.
→ → → wood was soaked 2 days ago, swollen.
→ → → → hinges made of steel. lock clasp made of iron. Chest weighs 12 lbs and 40 grams when lifted.
→ → → → → inside is 8 kg of gold.

"we're going to need room keys after the read aloud text, right?"
Ok, but consider, what if we didn't have room keys at all?

But alright, assuming room keys:
-Rooms missing a key entirely. It's not even 31. Tenth Kitchen, just Tenth Kitchen. (Yes we need a lot of kitchens, all similar and described in exquisite detail)
-Map keys referring to nonexistent rooms, obviously
-Skip a number, or a few. There is no room 52 on the map or in the entire adventure.
-There should be at least three different versions of room 51, all contradictory. (Was contradictory DM text mentioned already? A room needs to have a lich in it, and no lich in it at the same time.)

A few other ideas:
-NPC descriptions and NPC stats need to be at least 100 pages apart.
-This cannot use a gold or silver standard. It needs to have an electrum standard (a fact never mentioned anywhere), and a very elaborate money conversion system, with e.g. bronze coins worth 1/7th electrum. They're not referred as electrum or bronze of course, but with made-up names like "Third Dynasty grucats" and "sifs".
-Rename stats and classes for no reasons.
-Use a mix of multiple measurements. Feet and meters, lbs and kg and stones and slots.
-Somewhere in the dungeon there should be a portal to a whole world the DM must improvise out of thin air. What's the Grobulon Dimension? Fuck if I know, but it leads there. Or maybe a portal to literally anywhere. Take your setting's planet's map (you have a painfully detailed world map, right?) and roll coordinates at random! Oh, and 1d1000 years back in the past. Hope you have a detailed setting timeline.
-A magic item that mind controls the characters to railroad them into following The Plot.
-The big baddies use odiously powerful magic items (or tantalizing, really interesting items used in horrendously boring ways), which explicitly don't work (or outright disintegrate) if PCs use them, justified with obscure bullshit backstory reasons (if at all).
-OOOOH I FORGOT. Very important: the adventure needs a NPC that hires the party, then inevitably betrays them.
-(Also lots of traitorous monsters, and doppelgangers, so your players become paranoid forever.)

Every detail of any room has a separate DC to notice any particular detail, and a further DC to a history or knowledge check to determine in universe connections. Despite ostensibly a beginner dungeon, the challenge of the checks seem to have been calculated to be impossible for all but a maxxed-out optimised build level 20 character.

"The room has a stone flagged floor coloured dark grey (DC 15 perception to notice floor) laid out in the pattern of the sacred circles of Glarenthil (DC 30 arcana check to recognise this), it is a different stone from the walls (DC 20, Dwarf characters who took the stonemason background from appendix C may add 3 to their check) showing that the floor was added later and belies the history told to the characters by the hidden scroll from room 1_C (DC 17 intelligence check to make this connection). A rusted chandelier swings from the ceiling (DC 35 perception to notice chandelier), the trap on the chandelier (8d6 lightning damage, dexterity save 20 for half) will immediately be triggered unless the chandelier is noticed, and power word to disarm it is spoken (DC 25 Arcana Check to recognise the trap is disarmed by a code word DC 40 History (Backstory Castle) needed to recognise the disarm word is the name of the Elf Queen Hyacinth). Across the room is a bookcase (DC 18 Perception to notice bookcase) filled with dusty tomes (DC 20 arcana check to notice that there is nothing of value in the books which contain 19 cp worth of paper each). Glathnark the Mocron-Reiru Assassin is hiding behind the bookcase (looking behind the bookcase and succeeding in a dc 50 spot check will reveal the assassin) and will spring out and assassinate (Death attack auto hit, crit on 3+) the last in marching order if the characters pass through the room to the door on the other side (DC 15 perception to notice other door)."

Half of the tables' results are some form of "dm choice" or "pick other two". Especially the most important treasures are not defined.

I realise this is much too late, but an addition that would do wonders is if all the monster boxes vary by rule editions too. Darkbone Haunters are 3rd edition d&d, mocron-reiru are written with OD&D notation, each monster has an AC value, but you don't know if it's ascending ac or descending, some monsters use Target20 and some use THAC0 to attack. Some monsters have attack matrices (but not the same monsters that have OD&D notation).

The only thing that is easy to find is a full page near the front of the adventure explaining all of the spells which are NOT ALLOWED in the dungeon. "Techt has enchanted the area so that teleporting instead does 2d8/2 damage to the caster. Techt has enchanted the area so that divination give incorrect information most of the time."

In the adventure, or maybe a hangout, add a few new rule subsystem that emulate/simulate something in very minute and unnecessary details (such as mass breathing or camp-building, for example), and then insist that these rules variations are mandatory for this adventure, so players will have to learn and master them. Add one or two places in the adventure where these rules are absolutely essential for the victory, and then never bring them up ever again so at the end players will waste time learning and memorizing them.
Subsystems, of course, are either very bland or exceptionally intertwined or both.

Half of the place has careful encounter balance to ensure there's no actual risk of permanent player consequences (the horror!), and then the other half is full of "fuck you, you die instantly with no countermeasure" challenges, and there's no way to tell which is which amidst all the complex backstory and wording.

Many encounters have notes about adjusting the "difficulty", like "if you think your player are steamrolling the dungeon you can add 1d6 goblins in room 2B and 3d6 Mocron-Reiru in room XII"

No matter what the players are doing, no matter how innocuous the situation is, randomly stop and ask them to detail their actions and locations exactly like they're about to be ambushed.
Sometimes they are. Sometimes it's a really bad trap. Sometimes you just let out a sigh and say "ok, let's move on". There is no pattern.
Also, combat is officially played out on a grid but the DM must ignore the grid and run it as theater of the mind without telling players. Or vice versa. Lots of "how long are your arms" questions and wankery involving what exactly a flail does to a shield in 1400's Brittany, France.

What about treasures? There ought to be just enough gold to make it feel like someone is making fun of you. 1d6 electrum pieces (electrum because it's a pain to manage) in a trapped chest?
A chest in every room with the same trap and the same reward perhaps?

Specifically detailed ending script, maybe with multiple options. But they're shitty, like a video game "which of these three levers do you pull" ending.

A long-winded and overly detailed explanation of What Happens Next that expects you to build your entire campaign around this dungeon. Intricate details about the plans of the various NPCs that leave no room for player interaction. The adventure railroads the GM.

Similarly long-winded "where to place this dungeon" section that covers where, exactly, down to the hex or grid coordinate, where you are SUPPOSED to place this dungeon in fifteen different published settings including specific NPCs who have But Thou Must'ed the PCs into entering the dungeon. Absolutely no advice on integrating it with a homebrew setting.