Director - John Schlesinger

As want-to-be gigolo Joe Buck (Jon Voight) and crippled scam artist Ratso (Dustin Hoffman) cross a street in New York City, a REAL NYC taxi cab driver who ignored all the "Street Closed for Filming" signs drives through the scene.
Obviously this wasn't scripted and Hoffman's response and actions were all improvised, in character, as a result.