In the prologue, a corpse of an assassin can be found in a haystack, and when you approach it, Geralt says "Hm. Guess they'll never learn." A reference to Assassin's Creed.
In Roches garrison there is a man with the number 13 tattooed on his neck. He doesn't have a name, just thirteen. A nod to the song "thirteen" by Johnny Cash.