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.
Revealing mistake: Half the time the Colt 1911s in this movie are fake. In the scene where Tom Hanks is killing all the mobsters in the rain and it's in slow motion, if you watch all the guys with 1911s firing, there are muzzle flashes, but no slide movement and all the hammers are decocked. You can also see this in the scene where Hanks kills Paul Newman's son. It's interesting though, because the muzzle flashes aren't added in later - if you watch the making of it on the DVD, you can see Tom Hanks firing one of the guns - they're basically shooting smoke and a reddish, somewhat fire-crackery looking red flame.
Plot hole: When Sullivan shoots Connor Rooney at the end, he flicks the door and we see the dead body. However the wall and some of the floor have loads of dried blood on it when he was only shot seconds before.
Revealing mistake: When Peter and his mother are being shot, note that Mike Jr., standing outside the house, hears the sound of the shots just before seeing the flashes. The slight delay between the flash and sound is probably pretty close, but he should have seen the flash before hearing the sound of the shots.
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. (00:09:20)RLN
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.
Continuity mistake: In the scene where Michael Jr. is riding home on his bicycle (before his mother and brother get shot) he drops his bike and runs to the house. A few minutes later when Tom Hanks is driving off with Michael Jr., it shows his bike. The bike has snow covered on it, but it is not snowing, the snow has stopped.
Continuity mistake: After the night scene of heavy rains, you would expect at least some of the snow to melt the next morning. But if you look closely, the snow mounds in front of the Sullivan home are identical to the shots in the first few scenes, before the heavy rain.
Revealing mistake: When Mike and Mike Jr are driving down the road after Mike shoots Rooney's messenger the lights of a commercial plane flash twice in the night sky.
Continuity mistake: When Hanks dies at the end, you can see his left hand laying on its side, but in a wide angle shot of the same scene, you see his hand laying on its back.