Factual error: Tom Hanks is driving his car over a bridge in downtown Chicago in 1931. In the background is the elevated train structure. An aluminum bodied train passes on the trestel in the background. This aluminum bodied train is of 1980's contruction. In the 1930's the train cars were of wood construction and painted brown. I remember riding them for they were still in service in the 1950's.
Factual error: When Tom Hanks first drives into Chicago among all the 1931 era cars, a giant, modern, swing-arm, stationary crane is reflected in his car window. Also, The reflection of the Sears (Willis) Tower is seen in the car window as they drive along during this same moment, which wasn't started until 1970.
Factual error: In the scene where Michael is going to get John Rooney's jacket, we are introduced to the character Conner Rooney. Notice that Conner is smoking a filtered cigarette. The movie takes place during 1931, but filtered cigarettes were not introduced into the market until 1936.
Factual error: Some of the movie was filmed in the Conrad Hilton Hotel in downtown Chicago. Only in 1931 this hotel was named "the Stevens". Conrad Hilton purchased this hotel in the 1950's. Look closely and you can read the name "Conrad Hilton" written in script lettering. The lettering is out of focus on the wall in the background so you have to know what to look for.
Factual error: Tom Hanks with his son pulls into the small town, parks diagonally at the curb, tells his son to stay in the car and honk if he sees trouble. He walks to a building, the curb of which at the corner is shallowed to street level for handicap access. Handicap curbs are a modern development, certainly not around in the 1930s.