Continuity mistake: When Ratso finishes heating a meal he tells Joe on his bed reading a comic, to sit at the table and eat because it is hot. Joe gets up and puts his right hand on the chair - in his left hand is the comic he's reading. But in the next shot as he rests on the seat his left hand touches the table and his right hand now holds the comic.
Trivia: 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."