Mosquito Bites (dramatic irony)
Perhaps the ultimate symbol of Hutter as a stubborn rationalist comes when he writes to Ellen that he has two mosquito bites on his neck. Despite the fact that he read the book about vampires in the inn, witnessed the freaky likeness between the man driving the horse-drawn carriage to the castle and Orlok himself, and had Orlok lick blood from his hand, Hutter refuses to acknowledge that the occult reality is even the slightest possibility. This is a classic example of dramatic irony: we know a lot more than the character does.
The Mastermind, The Mad Man (situational irony)
Knock is the most deeply ironic character in the film, and Murnau's sole vector for social commentary. He's obviously a madman, but he is generally accepted by Wisborg because his business pays its real estate agents well. Additionally, he is savvy enough to find a way to bring his master, the Nosferatu, to Wisborg, but so stark raving mad that he spends his time in his jail cell eating flies.
The Virgin Seductress (situational irony)
Ellen, in the end, has to be the one to seduce the Nosferatu in order to keep him alive until sunrise. The irony, of course, is that the story has made it abundantly clear that Ellen is just about the purest woman imaginable, and so we are surprised to see her skill at seduction. The implication, here, is that she is a virgin. Thus, the virgin's sole act of seduction results in her own swift death, with the irony resolved by way of the virgin's sacrifice.
The Abandoned Building (situational irony)
When Knock tells Hutter that Count Orlok is looking for an abandoned building in Wisborg, the proposition doesn't strike Hutter as particularly strange. Of course, this bizarre request is quickly swept under the rug on account of the fact that Hutter knows he will be compensated well. Were Hutter to note this irony, though, perhaps he never would have embarked on the journey that would invite terror upon his quiet little town.
