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Nightmare Fuel: ER
There is a season 8 episode with Abby alone in her bathrobe, when she hears a knock on the door, and she hears from the intercom that it's the pizza she ordered. She buzzes him in, and then opens the door to find out that it was the wife-hitter whose wife she had just saved. He tries to find out where his wife is, Abby doesn't tell him and he then leaves after a threat to call the police. She hears another knock on the door, and she stupidly unlocks the door (but leaves the chain lock on) and, it's still the hitter and he somehow forces himself inside and punches Abby until she falls unconscious.
Gant's death was extremely blood-chilling.
Mark being beat up in the men's room. Completely unprovoked and unexpected and to make it worse, at one point, it seems to be over, only for for the attacker to strike again.
Carter's stabbing. As he falls to the floor, he (and the audience) sees Lucy in even worse condition. That's right. As horrifying as the attack on Carter was, with only the aftermath, we instantly realize that whatever Lucy went through was even moreso.
Some of the gore can be a little unsettling at times.
Pratt in the trauma room bleeding out of a carotid tear. Basically he's puking up blood.
The entire confrontation between Abby, Luka and Curtis Ames, a patient with a stroke. Basically he proceeds to stalk Kovac for weeks, then proceeds to hold Abby almost hostage until Luka arrives, forces Luka to crush his hand in a vice, then kills himself on a rooftop. All while Abby is left wondering if he'll survive.