Revealing mistake: When Count Orlok has loaded the crates onto the cart, he climbs into the last one and the lid "levitates" into place. This trick is achieved by stop-motion animation, but the cart horses do not hold their position and shatter the illusion - their heads unnaturally jerk about while the lid is in motion.
Revealing mistake: When Hutter is writing his letter to Ellen in Count Orlok's castle, the paper that he is meant to be writing on is blank throughout the scene.
Revealing mistake: When Hutter is lying on his bed unconscious after being threatened by Count Orlok, you can see the 'unconscious' Hutter blink.