From wiki: One of the most famous scenes in the movie is when Peck puts his hand into the "Mouth of Truth" (La Bocca della Verità), a stone face in Rome that legend says will bite your hand off if you tell a lie. In the film, when he pulls his hand out it is missing, causing Anya/Hepburn to scream. He then pops his hand out of his sleeve and laughs. Hepburn's shriek was not acting — Peck decided to pull a gag he had once seen Red Skelton do, and did not tell his co-star beforehand.