When the ambassador seals the letter, he drips wax for the seal and there is a trailing drizzle of wax on the paper, but when Don Juan is handed the letter, the seal the firm and there is no trailing there.
In the scene where Leporello is shaving Don Juan, he clearly completes shaving the right side of Juan's face. But when Juan takes the razor to complete the job, we once again see lather on the right side of his face.
Duke de Lorca: You can see we have several interesting devices in this chamber. They have the tendency to break a man's will. If the first one fails, we try another until we find exactly the right one to loosen the tongue.