Factual error: During the car chase that leads to the tunnel, there are several freeway signs pointing to Murphy, North Carolina, and the film is set in Chicago.
Factual error: In the scene on the El where the cop gets shot, you can hear the train engineer saying, "Balbo, next stop"; this is also said later on the police radios, when they say there is an officer down at the Balbo station. Two problems: 1) There is no Balbo station. 2) Even if there were, the station they start from is at Lake street (looks like the main transfer station at Clark and Lake), and the train pulls out going north. However, Balbo is south of Lake street.
Factual error: When Dr. Nichols leaves the damaged elevator, and Kimble later exits (the infamous arm in the door shot), neither would be possible as the elevator was emergency stopped by Nichols and can be seen positioned just below the floor. The elevator doors would not line up with the floor doors, and both sets would need to be manually opened.
Factual error: In the train crash scene there are many mistakes. 2 locos are pulling a hand full of cars, a small switcher loco would actually have been used. Oil journal bearings that haven't been used for decades. A flat car carrying logs, but there is no logging in the Chicagoland area. The second loco for some reason derails. In reality the train would have just pushed the bus a few hundred feet with no derailment. The shed explodes. The 200 ton loco just slides along the soft ground.
Answer: When Richard changes the diagnosis, the first thing he writes down is "AO" which is medical shorthand for aorta. Many people who have medical degrees and saw the movie speculate that Joel had an aortic tear. This would cause blood to flow into the chest cavity making it difficult to breathe and with the impact from the crash it could have caused the fatal injury. An aortic tear requires immediate surgery and by changing Joel's diagnosis, Kimble was able to save his life.