Continuity mistake: In the scene where the star and the professor's assistant are discussing what type of beast was involved in the attack, they are having sandwiches and coffee. One shot shows her front and his back, the next shot shows his front and her back. As the conversation and shots go back and forth, she's holding a sandwich in one and a coffee cup in the other. This happens numerous times and there is no lull in the conversation that would allow the switch.
Continuity mistake: When the Beast first tears down the streets of New York, a lone police officer empties his service revolver at the beast before being eaten. A few minutes later, a group of a half dozen police officers pump their shotguns before firing at the beast - the officer who was already eaten after firing his revolver can be seen alive in the far left of the screen as one of the officers readying his shotgun.
Factual error: A remarkable error. Late in the movie there is a shot where extremely large, perfectly round blood-stains are shown on the street as soldiers are shown giving chase to the wounded monster. These ignore basic physics. Whilst the source of the blood may be huge, individual blood drops would not have been proportionately larger.
Continuity mistake: In the scene where George falls into the chasm after seeing the beast for the first time, his gloves change from being inside out and the right way around several times. Even when he is being rescued the gloves can be seen to show the leather side and then the fur side, alternating as the scene progresses.