Character mistake: Kimble has just finished talking on the phone with his lawyer when the cops hear the noise in the background. They figure out the sound is that of an El-Train. They then start to list the different places that have an El. One of the cities they name is Milwaukee - there has never been an El-Train there.
Character mistake: After Kimble jumps from the drain pipe over the waterfalls, the scene goes to Gerard standing in the pipe telling his crew to leave, then they all leave without picking up Gerard's gun that Kimble threw down next to him.
Character mistake: The investigators say that the prints found in Sykes' apartment belong to Kimble after dusting for them, making it clear they didn't use any equipment for fingerprint analysis or testing. No forensic investigator would confirm who the prints belong to until they run a thorough analysis. It seems as if they were basing the fact that they belonged to Kimble because they traced the call to the apartment, rather than acknowledge that the prints could belong to someone else.
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.