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
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
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