Revealing mistakes: After Esmeralda gives Quasimodo water (he's still chained down), the nose putty on Quasimodo's left cheek is coming loose - there's a noticeable gap between Lon Chaney's face and the nose putty used to make the fake cheek.
Continuity: The hair on the back of the Hunchback's hands appears early in the film and later disappears.