Factual error: Asystole (the lack of cardiac activity, also known as the 'flatline') is not treated with defibrillation (electric shocks) but with alternating doses of the drugs epinephrine and atropine.
Factual error: When they jump-start Rachel's heart, she's wearing a bra. As medical students, one of the first things they would have learned is CPR and the use of defibrillators. They should have known to remove things like that or the electricity would have left burns on her chest, particularly where any metal touches.
Factual error: Nelson crosses a vacant street and enters a subway station where he encounters the child ghost that is haunting him. The corner exists, it is called "Six Corners" because three streets come together, and is in Chicago's Wicker Park Neighborhood. However, the train station at that corner is an elevated train, not a subway - this scene is filmed at Damen, Milwaukee, and North Avenues. The real Six Corners is Cicero Ave, Irving Park Rd, and Milwaukee Ave.
Factual error: Julie Roberts can't intubate because the trachea is "tight." This is laryngospasm, the vocal cords clamping shut. If so, neither mouth to mouth nor mask will work, but more importantly, no heart beat equals no blood to muscles, and they would relax and be open anyway. I think they just didn't want to fake a half tube coming out of Sutherland's mouth.
Chosen answer: No hallucination. It was Halloween night, so everyone was acting a little crazy. That's why one of the med students shows up in a skeleton suit. Going further, the 'hum' of the bikes were used not only to throw the viewer off as the same humming noise Nelson describes with coming back from the dead, but to confuse Nelson with death and reality and these new sensations/feelings he is experiencing since being brought back to life. The cyclists are used, in a sense, to scare Nelson and viewers. The same 'hum' can be heard when Nelson first finds Billy Mahoney in the dingy tunnel.