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In the Blood and Wine expansion, the quest "The Warble of a Smitten Knight" ends with Geralt lifting a curse laid upon a character named Vivienne, who was partially turned into a bird. In a cutscene prior to lifting the curse without Guillame's help, Geralt warns her that once she becomes human again, she may only live for as long as she spent living with the curse, which is seven years.

Afterwards, waiting seven in-game years after completing the quest will allow you to find Vivienne having collapsed and died inside Yennefer's room in Kaer Trolde in Skellige, although this position is slightly glitched and Geralt can still talk to her as if she were alive. According to the game's lead quest designer Philipp Weber, this buggy appearance was caused by the cloth physics on her dress as dead characters in the game are normally static models, and due to this being one of the last secrets added to the game prior to the end of development, they were unable to fix this in time for release. Coincidentally, this Easter egg was first teased by the developers and discovered by fans in 2022, seven years after the game's release.
Contributed by MehDeletingLater
In the Novigrad, the side quest "A Feast for Crows" is a reference to the "A Song of Ice and Fire" series by George R. R. Martin, both in that the name of the side quest is directly lifted from the fourth book in the series, and that it references events from the first book "A Game of Thrones". In the small isle of Kaer Almhult, one of the sky cells has a dead dwarf inside as a reference to when Tyrion Lannister was imprisoned inside the Eyrie's sky cell by Lysa Arryn. Geralt will say upon examining the body: "Sky cells... Nice idea for a prison without bars. Shame he didn't know how to fly".
Contributed by ProtoSnake
During the Fists of Fury: Novigrad fistfighting quest, Geralt encounters several characters with references to fictitious and non-fiction persons.

The first person Geralt fights is Georgius "Piledriver" George, a reference to Gorgeous George (real name George Wagner) the former American wrestler and character Gorgeous George, an unlicensed boxer, in the Guy Ritchie film Snatch.

Before the fight with Georgius George, Geralt is asked to throw the fight, a possible reference to Gorgeous George's notoriety for cheating during matches. The fight taking place at the Golden Sturgeon, a tavern, may also be in line with Wagner's real life alcoholism late in his life.

Archibald O'Neill's last name is spelled with one "L" on a notice board flyer. O'Neill is a reference to Mickey O'Neil, a character portrayed by actor Brad Pitt in aforementioned film Snatch.

Durden the Tailor, the last fighter, is a reference to Tyler Durden, a main character in the movie Fight Club also portrayed by Brad Pitt.
Before the match Durden gives a comment that is an excerpt of the speech given by Count Adhemar to William in the movie A Knight's Tale:

"You have been weighed, you have been measured, you have been found wanting. In what world do you think you can defeat me?"

Master Claytop's name and rigging of matches is a reference to Bricktop, a character who also has matches fixed in the film Snatch.
Contributed by Kakariko Kid
During the quest "The Tower Outta Nowheres", Geralt encounters a wizard trapped behind a magical door inside a tower. The wizard claims he was trapped there by the tower's Defensive Regulatory Magicon (DRM), which locks down the tower if anyone but the original owner tries to enter it. The tower's previous owner wrote a treatise on how to remove the defenses called "Gottfried's Omni-opening Grimoire" (GOG).

The initials "DRM" are a reference to Digital Rights Management, which is used by game publishers and platforms to prevent a game's original purchaser from freely sharing digital copies of it over the internet. The initials "GOG" are a nod to the website Good Old Games, which was founded in 2008 by The Witcher 3's publisher CD Projekt. It's famous for selling DRM-free copies of games and other software so that users may freely backup and share what they purchase.

Later on, while Geralt and the wizard are escaping, the defense system notes "disturbing indications of freedom". This is likely a jab by the game's developers at aggressive DRM policies, and echoes a common criticism that DRM restricts end users from freely controlling the software they purchase.
Shortly after the game's launch, one player discovered an easy way to earn money early in the game. They could kill cows, sell the hides, meditate to replenish the herd and repeat as desired. When CD Projekt Red found out about this, they decided to stop players from exploiting the method with the "Bovine Defense Force Initiative" patch. If Geralt kills too many cows too quickly, a powerful monster would then appear which can easily kill low-level players.
Contributed by raidramon0
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After completing the side quest "Contract: Monster in the Cemetery", back at the cemetery near Lindenvale, you'll come across two angel statues that weren't previously there. If you turn your back from them and turn back, you'll notice that they've moved. This is a reference to the Weeping Angels from the television series Doctor Who.
Contributed by KnowledgeBase
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There is a village called "Lurtch" located in central Velen. The description text mentions it being the former location of the "Evves" family estate, the name of the former lords wife was "Mortilanca" and the name of the former family butler (and inspiration for the villages' name) was "Lurtch".

This seems to be an obscure reference to The Addams Family, particularly Morticia Addams and their butler, Lurch.
Contributed by DarkChojin
There is a chance that a human enemy will say "Your mother was a hamster and your father smelt like elderberries", this is a reference to a scene in Monty Python and the Holy Grail.
Contributed by generalcodeblue