The Rabbit of caerbannog is a legendary creature in the Monty Python movie (Monthy Python and the holy grail). You can find this (snow hare) in the game at the entrance of a hidden cave near Benek, the location of the side quest (The Truth is in the Stars) in Velen.
You can only kill it by bomb just like the movie.

In the Blood and Wine expansion, if one uses mods or cameras to access the top of Mount Gorgon, they can find a photograph of the team between two pillars.

Novigrad is similar to the city of Gdansk in Poland. Historically it's been an independent city between 1920 and 1939. It also bears similarities of medieval trade city of Novigrad in modern Russia.

The achievement called (Let's Cook) requires to learn 12 potions is a reference to the Tv series Breaking Bad in which the protagonists "cook" meth learning new formulas.

The achievement called (Hasta La Vista) requiers to kill a frozen opponent with a crossbow bolt and is reference to Terminator 2 Movie in which protagonist shoots a frozen Terminator and says Hasta La Vista Baby.

The achievement/trophy called (Fast and Furious) is awarded for winning all horse races in the game is reference to movie Fast and Furious.

In the Blood and Wine expansion, right before leaving The Land of a Thousand Fables, a Will o' the wisp can be followed to a bonfire. When lit, Geralt will sit and the game will automatically save. If Geralt dies in the vicinity, the game will load the last save which will be at the bonfire.

In the ruined fortress of Kaer Almhult, there exist sky cells, i.e. prison cells with sheer drops on one side in place of a wall. One of the cells contains the body of a dwarf. This is in reference to the incarceration of Tyrion Lannister in such a cell, in the first season of Game of Thrones. Upon examining him, Geralt says "Sky cells... Nice idea for a prison without bars. Shame he didn't know how to fly."

During the quest Family Matters, a portrait must be removed from the wall that is of the baron and his wife. It bears a striking resemblance to Eyck's Giovanni Arnolfini and his Wife.

At some point during the quest Turn and Face the Strange, you can enter a portal only to enter a small room with nothing but a bloodied crate in the center and a chest containing a Cake nearby.

During the quest The Great Escape you will find a corpse. Upon examining it Geralt will say "Heheh. Tomb raider. Seemed this one liked elven stuff".

In the Blood and Wine expansion, a knight during the quest Knight for Hire, may say "remember witcher - history is wrought by those who can hang heroes", referencing a famous quote from the film: "Historians from England will say I am a liar, but history is written by those who have hanged heroes".

In a cave southwest of Ard Skellig, three farting rock trolls can be heard farting inside named Jesse, Joe and Dodger. They're a reference to three popular Youtube gaming personalities and voiced by their namesakes:
Jesse Cox, known as YouTube user OMFGcata
Joe Vargas, known as YouTube user AngryJoeShow
Brooke "Dodger" Lawson known as YouTube user PressHeartToContinue

The name of whole series of the Fists of Fury quest is a reference to a classic Bruce Lee martial arts film.

The Night of Long Fangs, a main quest from the Blood and Wine expansion, may be seen as a reference to the Night of the Long Knives, a purge that took place at the beginning of Nazi Germany to consolidate Hitler's power.

During quest Spoontaneous Profits you can find a note: A Ra'mses Gor-Thon is mentioned in Smigole Louis Serkis Cookbook, a reference to the famous chef, Gordon Ramsay.

When fighting bandits sometimes they will say (I'll rip off your melon and ♥♥♥♥ down your neck) a reference to Duke Nukem.
https://www.youtube.com/watch?v=SKV87994GH4

Guards and villagers in Skellige will sometimes shout (This is skellige!), a reference to the famous line (This is Sparta!) from 300.

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