If you ask Alexa to tell you a very (for any number of verys) funny knock knock joke, she will say, "Sorry, I don't know any jokes about Renfield."
Knock in Nosferatu (the film) is the character equivalent of Renfield in _Dracula_. I think.
I have been googling in search of anyone else who has encountered this joke (well, first I just wanted someone to explain it to me because I didn't get it). Let me know if you can find any indication! I have found other lists of easter eggs.