After returning from Florida, Ray wears a Tampa Bay Buccaneers hat. At the time the movie was filmed, the Buccaneers and Chicago Bears were division rivals.
Darlanne Flueguel appeared in another cop movie from MGM a year before in which she played a love interest of William Petersen's secret service character in To Live in Die in LA directed by William Friedkin who directed The French connection in which the El car chase is inspired by that film. Also Billy Crystal and William Petersen also appeared in movies like Manhunter and throw momma from the train with Kim Greist as their love interest as well as Darlanne Flueguel in this and To live and die in LA.
During the chase through Chicago, Ray Hughes radios that the vehicle is traveling south on I-190. That stretch of tollway runs East/West not North/South.
In the scene in which Gregory Hines and Billy Crystal are discussing their shooting accuracy at the get away vehicle with the men who tired to rob them after they first arrest Snake, Hines' character says he hit the windshield with all six shots. However, there are only 5 bullet holes in the windshield.
In the opening scene, Hynes and Crystal are in a Chicago park at Christmastime. All of the trees in the park are loaded with green leaves - not something you'll ever see in Chicago in December.