In sourcebooks: Dungeons & Dragons, Monster Manual, Deities and Demigods, Creature Catalogue, Monster Mythology, Elder Evils, Fiend Folio, Heroes of Horror, Savage Species, Primal Power? The Will and the Way, Gold and Glory, Elminster's Ecologies?
Ascended Meme: The "green flame" Running Gag from the Acquisitions Incorporated games has become popular enough that the Fifth Edition Sword Coast Adventurer's Guide includes a cantrip called "Greenflame Blade". In addition, the adventures "Out of the Abyss" and "Curse of Strahd" have a mace that emits green flame and a gateway filled with green flame, respectively.
Bowdlerise: The game was significantly changed in the transition from First to Second Edition to avoid the Satanic Panic that had scorched the game's reputation, and was heavily redesigned for full family-friendliness. Demons, devils, and PC assassins were out, artwork was restricted to PG-rated, and a Dragon article by James Ward explained the official policy of "avoid the Angry Mother From Heck."
Character Tiers: A unique variant. The classes are tiered not on their power, but on their versatility (and thus ability to solve traps, social encounters, and other non-combat stuff given by the DM), then broken up into how well they can do that. Fighter are traditionally low tier not because they are bad in combat (though he may be), but because he is complete dead weight outside of combat - a Fighter may be good at some physical activities like climbing and balancing, and may also be some use in social situations by intimidating people, but that's really it. The Rogue meanwhile is higher because she may work at social encounters, traps and combat, but she isn't fantastic at them. Spellcasters like Wizards and Druids are nearly always high/top tier, the former because their magic gives them incredible versatility depending on what spells they have prepared, the latter having nature-themed versions of many of the Wizard's magic options plus Voluntary Shapeshifting to back it up. invoked
Gary Gygax had several, including mushrooms, various shades of the color purple, H. P. Lovecraft, his extensive vocabulary, polearms, and infamously difficult adventures/dungeons. On the unfortunate side, problems with ranged weapons from slings to wheel-locks. Only in 3.x Edition was his fascination with polearms finally dropped... to be replaced with some new developer's spiked chain fetish. These things wormed their way everywhere, even underground.
On the note of 3.x Edition, former Wizards of the Coast employee Monte Cook enforced Linear Warriors, Quadratic Wizardsmuch more than in previous editions. Martial classes were either mechanically schizophrenic (monks), ridiculously generic (fighters), or straight up terrible (3.5 samurai, which was a straight port of the 3.0 samurai without any reworking). Caster classes were overpowered due to both the number of overpowered spells they could get and the introduction of metamagic feats. Wizards in particular were the worst about it. With 5E, Fighters are far more viable mechanically (retaining a lot of the tools they gained in 4E, for instance being the only class to keep the action surge feature) and Concentration rules for most spells limit the number of magical effects casters can keep up at one time, making the playing field much more even.
Gender-Neutral Writing: Uses Take a Third Option to this trope. Instead of awkwardly avoiding pronouns or always using one gender or the other, each class has an example character, and the classes description uses pronouns that reference them.
Idealized Sex: According to the Dungeons & Dragons 3.5 module "Book of Vile Darkness", only evil people can have a sadomasochistic sexuality. Also, all sadomasochists have evil superpowers sexual masochism and sadism are evil superpowers in this setting, and sexual masochism is defined as being the same thing as the trope Combat Sadomasochist.
Lava Adds Awesome: Invoked by a number of spells and magic items, such as "Vulcan Bomb," which hits a target with a stream of lava.
Life/Death Juxtaposition: In the Dungeons & Dragons cosmology and those derived from it (e.g. Pathfinder), the positive energy plane is the origin of all life and provides energy for magical healing and resurrection while the negative energy plane is the origin of The Undead and its energy weakens or outright destroys all living matter.
Fifth Edition lists the good and neutral deities up front in the character creation section, while setting the evil gods firmly in the 'know your enemy' part of the book. This, of course, has no effect on some players and DMs, who create all-evil campaigns frequently and with panache.
The D&D based RTS game Dragonshard has a campaign for the humans and the lizardfolk, but not for the Umbragen.
From the Immortals boxed set, player controlled PC Immortals are forbidden to be from the Sphere of Entropy, because creatures from that Sphere are all evil. All Entropy Sphere Immortals are NPCs.
The author of a Dragon article on the "Death Master", a necromancy-themed Non-Player Character class for 1st Edition AD&D, introduced it by stressing, thusly, that it was designed for NPC villains only:
"If I ever run into a player character Death Master at a gaming convention, I may turn Evil myself."
Incantifers, from the Planescape Monstrous Compendium Appendix II, are creatures that used to be human beings. They were changed by magic so that they can absorb magic and don't need to eat, breathe or sleep (among other powers). They have evil tendencies and Dungeon Masters are warned not to allow PCs to undergo the incantifer-creation process.
Dragon magazine #50 article "The Glyphs of Cerilon". One of the Symbols (clerical/priest spell) in the article causes damage to anyone of Evil alignment who tries to pass it.
Several spells do this, such as the mage/wizard spell Antipathy and the cleric/priest spell Forbiddance, which can be used to enchant just about anything to make a creature of any given alignment not wish to approach.
The Forgotten Realms city of Silverymoon has a variety of powerful magical wards, one of which bars entry to creatures with an evil Character Alignment who belong to a long list of species. Evil humans and demi-humans, however, are free to come and go.
Painting the Medium: The Leprechauns on page 60 of the 1e Monster Manual play around with the page headings. They also ride the giant leech to their left.
Purple Prose: All D&D books are written in a somewhat formal and archaic style, but 1st edition was probably the worst about it.
Retcon: Fourth Edition had a relatively minor one concerning the war between the Gods and Primordials.
Retraux: An "old school renaissance" has sprung up, with a number of retro-clones (OSRIC, Swords & Wizardry, Labyrinth Lord, Basic Fantasy, and others) based on Basic, Original and First Edition D&D.
Rule 34: Both the "Book of Erotic Fantasy" and "Encyclopaedia Arcane - Nymphology" (both third-party and unofficial as hell, but still). Fairly tactfully, thankfully. Before this, it was "The Complete Guide To AD&D Unlawful Carnal Knowledge". As a netbook, it got stuff from "quite in-character in a normal game" (blow-a-kiss-with-effects spells may be a must-have for a love goddess' church, finding out how long it takes to blow your money on the hookers part of booze and hookers is also usable) to "where'd I put my Brain Bleach again?".
Scunthorpe Problem: During editing, one book had a search-and-replace run to change "mage" to "wizard." Unfortunately, it also changed "damage" and "image" to "dawizard" and "iwizard."
Sequel Number Snarl: The various editions are titled Dungeons & Dragons, Dungeons & Dragons Basic Set, Advanced Dungeons & Dragons, Advanced Dungeons and Dragons 2nd Edition (originally on the book cover, though later printings left it off), Dungeons & Dragons 3rd Edition, Dungeons & Dragons v3.5 (also referred to as "3rd Edition Revised"), Dungeons & Dragons 4th Edition, and Dungeons & Dragons Fifth Edition. Which technically means that the "Fifth Edition" is actually the eighth edition.
Seven-Year Rule: Every time a new edition comes out, it is the worst thing ever. People also completely forget that the current edition, which you would be led to believe is almost perfect by the standards of everyone, was ridiculed just as badly. People also neglect the difference between the amount of content a newly released edition has and the amount of content the current edition with over a decade of supplemental material has, often complaining about the lack of options. The Internet has naturally multiplied this effect.
The stoneskin spell in older editions of Dungeons & Dragons.
Module C1 The Hidden Shrine of Tamoachan. A fighter can receive a scroll that gives him a Death Servant. At any time thereafter, if the fighter is about to be killed, the Death Servant will push the fighter to safety and accept the attack that would have killed the fighter. It will only do this once.
Spiritual Antithesis: Is this to the original Chainmail. Compared to Chainmail, where each player commands an army against another player's army, Dungeons & Dragons was — and continues to be — a cooperative experience with each player controlling just one customizable character rather than an army.
Super-Sargasso Sea: Unsoncy. The center of the plane is a rotating disk of debris that comes out of a singularity in the middle. Items lost on other planes of the universe end up here. The Immortal who controls the plane always looks here first when he loses a pair of socks in his washing machine.
Technician vs. Performer: Building characters to fit a concept or theme (like a minotaur ninja) with less-than-optimal mechanics (Performer) vs. building characters to be as mechanically powerful as possible with roleplay as an afterthought (Technician).
One notable instance is an article with explanations of some of the harder rules. The page states everyone is proficient with splash weapons, then describes an example with a character taking a non-proficiency penalty when using a splash weapon.
One 3.5 Prestige Class that fits this trope would be the Abjurant Champion; a Magic Knight class that grants a character bonuses to Abjuration spells (such as Shield). It mentions Mage Armour as being another such spell, seemingly disregarding the fact that Mage Armour is placed in conjuration.
'Armor' has this bug back from AD&D1 even though other school assignments were fixed by AD&D2. Most DMs, if asked, will allow the spell as either an Abjuration as a "protective barrier" and/or Evocation (Force subschool) as one more force field; either one fits better than "create/call stuff".
As of Fifth Edition, mage armor is now classified as an Abjuration spell.
Any time the flavour department decide to try writing about 'realistic' stuff, it falls prey to being Wonkish. You'll see a piece that realistically describes military tactics as they'd be changed by the presence of things like Ogres and Werebears, then go on to completely forget about things like supply lines and waste disposal.
Complete Psionics includes feats that make a character a descendant of the Mind Flayers. This completely ignores the process Mind Flayers reproduce by note they're sexless egg-layers. The only way illithid hybrids are created is "Face Full of Alien Wing-Wong plug a larva into host other than Medium humanoid".
Then again, they could be descended from the failed ceremorphosis of humanoids where the tadpole did not inject enough of a certain chemical into the host body and wound up resembling a normal humanoid but with the brain and diet of an illithid.
The "Apostle of Peace" class is required to take the crippling "Vow of Poverty" which disallows the character from owning almost any wealth. The picture of the class has quite a few magic items (which are very expensive) in it. May be justified if the character has an immediate need for the items, for whatever reason, but the expectation is that as soon as the need is gone the item would be sold and the gold donated or given to one's god. That being said, its actually a truer example of how many players actually play such characters.
The Ruby Knight Vindicator example character worships Saint Cuthbert, but the class requires Wee Jas worship (It suggests DMs should make versions for other deities the deity requirement, but it's officially just a suggestion).
In 3E, The Epic Level Handbook has a creature it claims even the gods can't stand against, but that seems questionable when that creature's stats are compared with some of the gods' stats in Deities and Demigods. Judging from the Deities and Demigods stats and the stats of the titular creatures of the book Elder Evils, the gods could easily crush the elder evils even though the latter's book's intro describes them as so powerful that even the gods would think twice before fighting them.
Although it's less the creature's combat abilities and more their existence that makes the gods nervous. Most of the Elder Evils are highly resistant or even immune to divine effect and one elder evil specifically mentions that its greatest powers only work on gods (since he was defeated by Asmodeus who is not a deity) so it's most likely that the Elder Evils have other effects that do not show up in the books because they would not affect a PC in any meaningful way.
Writing Around Trademarks: The earliest printings of the original boxed set call the race of little people hobbits, as one might expect from such a Lord of the Rings influenced game. Grumblings from the Tolkien estate led this to be changed to halflings (along with a couple of monster names, such as "Ents" becoming "Treants" and "Balrogs" becoming "Balors". This despite Tolkien himself taking the term "hobbit" from folklore, and "halfling" actually being a term he coined, apparently.
Conversely, when a third-party publisher puts out a D&D-compatible book or derivative game, they'll use something along the lines of "compatible with/based on the world's oldest fantasy roleplaying game" to indicate D&D without using the trademarked name.
During the Third Edition era, the d20 System Trademark License program allowed publishers who followed a legal agreement to create material that was openly compatible with the game.
You All Meet in an Inn: Generally thought of as the inventor of the idea that the player characters will form their team while staying at the same inn.
Advanced D&D and 3.X edition: When creatures from the Prime Material Plane travel to other planes of existence they find that magic (spellcasting and items) don't work the same way they do on the Prime. Some spells/items have different effects, some don't work at all and some backfire. On rare occasions, it's possible to use magic that can't be used on the Prime.
BD&D Immortals set. Some planes (such as triplanes - no, not the airplane) lack the extra dimensions that are necessary for magic, thus preventing anyone - including visitors from other dimensions - from using magic while in them.
Module I12 Egg of the Phoenix. In one of the mini-adventures, the PCs go back in time several hundred million years to the time of the dinosaurs. Magic was much more potent back then, so spells have double normal effect.
Sometimes it doesn't even take leaving one's own home plane. Magic works differently in Mystara's Hollow World setting than it does on the outside of the very same planet, courtesy of the Immortals using the inside as a "nature preserve" of sorts for cultures that would have gone extinct in the outside world and using their own magic to prevent certain mortal tricks that could upset their pet project from working.
Also, dungeons in which specific types of magic — such as teleportation spells, say — simply wouldn't work (or at least not work right) rather obviously for the sole purpose of keeping the game challenging for the players weren't exactly unknown especially in the early days of the game.