Throughout the series reporters make reference to the "DA" (District Attorney). The prosecuting attorney in each county in Illinois has the title of State's Attorney, not District Attorney. No journalist in Illinois would ever refer to their county's State's Attorney as the "DA."
Chicago Transit Authority 'L' cars and lines are shown as color coded. This scheme was not introduced until 1993, a year after when the show takes place.
The main story takes place in Chicago 1992. Trump Tower is clearly visible in some establishing shots. However, Trump Tower Chicago was not completed until March 2005.