Any similarities between monsters depicted in this book and monsters that actually exist are purely coincidental. That goes double for mind flayers, which absolutely, utterly, and completely do not exist, nor do they secretly run the D&D team.
Exalted: To emphasize the extreme secrecy practiced by students of the style, the description of the White Veil Style of Terrestrial Martial Arts insists that there is no White Veil Society. They are not a secret political force in the Scarlet Empire, and they certainly don't teach their non-existent members a highly secret style of martial arts revolving around fighting in plain sight without anyone, even the victims, realizing it...
As of comic 251, the White Veil Society (which, of course, does not exist) has not made an appearance in Keychain of Creation, and certainly not in any kind of storyline-important way. That would be just silly.
Referenced by name in Compass of Celestial Directions: Autochthonia, using it to describe how the system of Militate promotions in Estasia definitely is not hugely corrupt.
Also, "This mission will be lots of fun and involves no dangerous targeted retroviruses."
In Warhammer Fantasy Roleplay, there is no race of man-sized rat people, the so-called "Skaven", living in underground tunnels running throughout the Empire. They do not have advanced weaponry, specially-bred giant warrior rats, and warpstone, Chaos crystallized. Only insane people (and dwarves) make such irresponsible and absurd claims.
The humorous "Sacred Commandments of the Adeptus Astartes" related to Warhammer 40,000 feature this:
193. By order of the Inquisition: There is no such thing as the Inquisition, questioning this will have thou deemed heretic by the Inquisition.
In Werewolf: The Apocalypse, the Black Spiral Dancer scholar Writlish is reticent on the subject of Mockmaw, an ancient Black Spiral king. In Garou Saga, Writlish tells a fellow scholar that the Black Spiral Dancers have little memory of any Garou by that name. In Chronicles of the Black Labyrinth, he dismisses a reference to "Moch Maugh" as a possible place-name.
In the Infinity setting, the Yu Jing army most certainly does not have ninja in it, because the last of the ninja disappeared at the beginning of the 20th century. Ninja no longer exist. They certainly don't exist as an elite black-ops and assassination unit that takes out enemies of the State with high-powered sniper rifles and monomolecular-edgedkatana. That would be ridiculous.