Continuity mistake: When Kimble goes back to the hospital as a janitor to check the hospital computer, the lady working in the lab is wearing two different dresses. (01:05:15)
Visible crew/equipment: After Sykes shoots the Transit Cop on the "L" Richard pulls the emergency brake, and in the following shot facing Richard as the train screeches in flashes of darkness, the reflection of a crew member quickly becomes visible on the glass window beside Richard. (01:47:45)
Continuity mistake: Kimble dives from the top of the dam when Gerard traps him in the tunnel. But in wide shots of Kimble falling, there's no tunnel above the dam. (The tunnel scenes were shot in Chicago, but the location sequences were done at Cheoah Dam in North Carolina - which has no tunnels. The end of the tunnel from which Harrison Ford supposedly jumped was built on the back of a large flatbed truck which was driven out on the top of the dam for the scene).
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.