Continuity mistake: The policeman drives up to the house and parks his motorcycle facing the front door. He goes to the door to talk and then returns to his motorcycle and prepares to leave. When he gets back to the motorcycle, it is now parked facing away from the door. (00:08:05)
Continuity mistake: Doctor Leo sits at the desk and begins writing a letter to Alec Ritchfield. "My dear Alec", it starts. Fade to the finished letter, but you can easily tell that it's not the same letter, the handwriting being different - look at the little flourish below the M, not to mention the position of the text in relation with the Fog Island stamp. (00:08:25)
Revealing mistake: In the cave scene, when Dr. Edelman and Talbot come upon Frankenstein's monster in the mud, there is a skeleton lying beside the monster, which Dr. Edelman identifies as probably Dr. Niemann. The prop is an obvious lab skeleton, as you can very clearly see the circumferential line around the skull cap. Also, if you look closely in several shots, you can make out the hook at the top of the skull where it would hang from a lab stand. (00:24:35)