When Dustin Hoffman dies at the end of the film, if you look closely at his chest you can see him breathing.
The most famous line in this film is purely ad-libbed. Apparently a tough New York cabbie broke through and drove across one shoot, nearly running down Jon Voight (Joe Buck) and Dustin Hoffman (Enrico Salvatore "Ratso" Rizzo). Notice Voight's startled expression. Hoffman, still in character, slams on the hood and shouts at the cabbie, "Hey, I'm walkin' here. I'm walkin' here."