Composer Quincy Jones was so impressed by the performance of his musicians for the soundtrack of the film, that he requested to Twentieth Century-Fox and the producers of the film to give them on screen credit during the end credits of the film. The featured performers given credit on screen were (among others): Gerry Mulligan, Grady Tate, Jerome Richardson, Frank Rosolino, Clark Terry and the Don Elliott Voices, who were all popular jazz musicians at the time.
In the Dortmunder novels by Donald Westlake, Kelp always steals cars owned by medical doctors. In the movie, the license plates of the stolen cars have the letters "MD" as part of the plate number. (Kelp's reason is that doctors have expensive tastes, and their cars are usually loaded with the best options - leather seats, stereo, etc..)
The weapon they are carrying during the helicopter ride to and the assault on the Police Precinct is the Swedish 9mm SMG m/45. Developed during WWII. Still in use within the Swedish Armed Forces (nowdays in a very small number). Once used by American Special Forces because of its simplicity and reliability. Also manufactured in Egypt on license.