Charlie Trotter plays himself in the movie. He is a Chicago chef and, at the time of the movie, had no restaurants other than his signature restaurant in Chicago. Jules reviews his New York restaurant; presumably, he was either hired away by a new fictional restaurant in New York or has opened a fictional branch of his Chicago restaurant there.
After Julianne set up Kimmy to discuss with Michael at dinner the proposition of working for her father, Michael becomes upset and Kimmy burst into tears running down her cheeks, move to their kiss where her face is completely dry, then she pulls away from the kiss and she has tears again.
When Michael is sitting on the bench at the train station after looking for Kimmy, two men in uniform walk behind him. As Julianne finds him and is walking towards him, the same two men walk behind him again. And the newspaper in the guard's hand changes from left to right.
At the start of the movie in the restaurant the wine glasses have very little in them. Throughout the scene they have the same (realistic) amount until George takes a sip from his, when it suddenly has a lot more.
Jules is sitting on the floor and leaning her back against the door with a cigarette in her left hand. When Michael suddenly opens the door she falls backward and, in the next shot of her flat on the floor, the cigarette is in her right hand.
After Michael and Kim pick up Julianne from O'Hare Airport, they're heading north on Chicago's Lake Shore Drive. Kimmy swerves wildly to her exit. The next time you see the car, it's heading south on Lake Shore Drive with the Drake Hotel in the distance.
The boat ride magically teleports the characters to many different points along the Chicago River, from Lake Shore Drive to being alongside Union Station (trains in background) to Clark Street Bridge (Merchandise Mart in background).
In Chicago, Lake Shore Drive is bordered by Lake Michigan on the east and the city on its west side. However, in more than one scene when driving on Lake Share Drive, the lake is either on the wrong side of the vehicle, or it keeps changing sides of the vehicle.
Julianne has two pairs of sunglasses throughout the whole time she's at Kimmy's parents' estate, often wearing one pair while the other is hanging on her top shirt button. When she and Michael reach the gazebo and she takes off the sunglasses she's been wearing, one can see that she's holding two pairs.