Continuity mistake: In the flashbacks seen at the start of the film, Charles Nichols introduces Kimble to Alec Lentz. When this flashback is revisited later in the film, when Kimble breaks into Frederick Sykes' house, the words and actions of Nichols and Lentz are slightly different to what was seen earlier in the film. (00:04:15 - 01:24:55)
Continuity mistake: After the bus crashes, Kimble unlocks his hands and throws the keys onto the floor in front of Copeland. Two shots later the keys are lying in a different position when Copeland grabs them off the floor.
Continuity mistake: Kimble puts the worker's uniform on over his prison uniform, but when he takes it off to tend to his wounds, we only see him remove the worker's uniform, with no sign of the prison uniform.
Continuity mistake: When the Marshals go to arrest Copeland, they are in Whiting, IN. When the Sheriff's car comes through, it is an Illinois State Police car.
Continuity mistake: The prisoner starts foaming at the mouth and pulls the sharp implement from his sock, but when he is shot, he collapses back in his seat with no sign of the foam around his mouth.
Continuity mistake: When Kimble leaves the hospital and the detectives arrive, it's dark outside. However, when they're interviewing the nurse, you can see it's daylight outside.
Continuity mistake: When Kimble is on the moving train, Sykes mysteriously appears from a door when he wasn't there before. (02:00:00)
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.