Factual error: Chris Sabian tells them to "Cut the electric, cut it now." Shortly afterwards Danny calls Chris on a cordless phone. Cordless phones only work if the base unit is powered, so Danny wouldn't have been able to make the call. This happens a few more times when they turn the power on and then off again. (01:17:40 - 01:20:30)
Factual error: An ambulance would never be parked directly in front of a hostage scene, as shown at the end of the movie, because (a) the ambulance would have to arrive before the police, (b) that location puts both the medics and the ambulance itself in jeopardy if shots are fired, and (c) the ambulance would be blocked in by the numerous police vehicles and thus unable to quickly leave with a wounded patient.
Factual error: When Sabian views Niebaum's chair, you can see the bullet holes through the chair and into the desk behind it. The SWAT members were using 9mm MP5s, and they would use hollow point ammunition. Hollowpoints bloom when entering the body and do not pass through. Even if they used full metal jacket rounds, 9mm still wouldn't pass through a 250 lb man, a chair, and into a metal desk.