Deliberate mistake: Joseph (John) Merrick's speech was never as audible as shown in the film, and several operations were needed before he could speak at all. Even after that, Frederick Treves often had to "translate" what Merrick said because the majority of people still had problems understanding him. [Probably changed to simplify the storytelling, but still inaccurate].
Continuity mistake: The period of time this movie portrays about Merrick, was in real life a period of seven years. In real life, Merrick's deformations were not so 'big' at 21, and at the date of dying, they were larger than portrayed in the movie. In the movie his deformations never seem to enlarge.