The medical staff should be wearing medical protective glasses when they're performing the autopsy on the BMW Driver, one medical staff member confirms this goof when he squints when blood sprays on his mask as he drills her chest.
The roles should be reversed, since Robert Duvall is older than Ray Liotta, it would be more logical for him to play the Chief of Police instead of the hostage negotiator because the rank of Chief of Police is higher therefore takes a longer time to achieve that rank.
When the cardiac surgeon agrees to do the surgery he says "what are they going to do... disbar me...?" But doctors are not members of the Bar, this is a legal profession term not a medical profession term.
During the closing scene in which John is being sentenced in court he is found guilty for the "kidnapping and false imprisonment of [each hostage]." In reality this would not have been a single charge, rather a series of individual charges with a guilty or not guilty verdict in the case of each hostage taken. Moreover, "kidnapping" would not apply to most if any of the cases as such a charge necessitates asportation (the moving of a person or property illegally). Given that they never left the ER, false imprisonment would likely be the only charge.
When John attempts to kill himself, he pulls the trigger and the hammer falls, but the gun does not go off because he says the safety is on. When a Beretta's safety is on, the hammer cannot be in the cocked position and the trigger mechanism is disabled, thus the hammer could not have fallen and pulling the trigger would have had no effect anyway.