The Beast From 20,000 Fathoms (1953)

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.