The caps of Chicago Police Department officers have a distinctive checkerboard band, absent on those in the film.
Stuntman Dale Van Sickel is clearly seen instead of actor William Talman in the shot where Hayes Stewart jumps over the skylight during the rooftop chase.
A waitress at the Silver Frolics orders an Old Fashioned from the bartender, without (among other things), grenadine. She should know that Old Fashioneds are not made with grenadine.
When Kelly looks at his watch, the minute hand is at 27, but the hour hand is just short of the 11.