This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Rescue 911
This program contains true stories of rescues. ALL OF THE 911 CALLS YOU WILL HEAR ARE REAL
To elaborate: Every phone call has the voice of a terrified human being who, as they speak, is fearful that they're about to die, or that they're about to watch someone else die. Often they've just been the victim of a crime, or dangerous accident. Sometimes it's a random stranger calling for help for someone else. These are not actors, and this is not make believe. This is what people sound like when they're frightened, desperate, in pain, and unsure if they'll be alive much longer.
The opening credits, just before the music ramps up and you see the rescue personnel acting heroic, can be a bit scary, what with the disjointed black and white footage and ominous music.
Right around the same time as above, as the intro to the theme song is at its darkest and most ominous, you hear a woman on the phone sobbing, "Ohhh, I need an ambulance!" She sounds traumatized, and because the soundbite is inserted without context, you don't know why she needs the ambulance.
"Teen Hides From Intruders": Imagine being 13 years old and home alone, and all of a sudden, BAM! A scary chick (nicknamed "War Bitch") and her boyfriend break into your house.
"911 Christina's Call": A woman is stabbed at least 20 times while her daughter was on the line with 911.
"What did he beat her with?"
A mother is in the shower, and her three-year-old son innocently unlocks the front door upon hearing a knock—thus letting in a rapist.
In the episode with the crocodile in Africa, they don't actually say it quite yet...but you can see that the dad's arm is missing.
"Trailer Fire," where a woman is screeching about how her trailer home is on fire.
A man cuts his wrist with a woodsaw in "911 Woodshop Trauma."
"Do you have anything you can use as a bandage? Paper towels?"
"Nooo! Toilet paper!"
"Ah that won't work..."
"Bathtub Seizure." Just the line, "This kid is going to see his mom die."
"Runaway Boxcars," and the thought of what was going through the man's head as the cars hit him.
"Cave Divers," wherein four divers decide to explore a cave in a freshwater spring without proper training. One diver swims out, but the other three are trapped in the cave. Two throw off their gear and swim up to an air pocket, but the third doesn't make it. The person rescuing them mentions that when people are drowning in caves, they often throw their gear off right as they drown, and that he finds the bodies floating above them most of the time.
"Chocolate Chips," as it deals with a nut allergy. Nut allergies themselves are VERY freaking scary.
"Roller Coaster Rescue," where a teenager gets trapped and dragged under a roller coaster. You can see his leg hanging down in the recreation.
The episode where a convertible accidentally drove right under a semi
"Brush Fire Rescue," where a firefighter gets accidentally run over by a brush truck (a one-ton pickup with a water tank on the back). The paramedic responding was a friend of the firefighter, and in the one-on-one interview, he says, "I was afraid he wasn't going to make it." Paramedics run the risk of responding to friends in emergencies, but the tone of the medic's voice and the look on his face says he was truly scared that his friend was going to die.
Even worse, the firefighter's mother was the fire department's dispatcher. During the episode, she continues to enquire about the injured firefighter's status. The continued response was "10-9, dispatch." 10-9 is code for "I can't understand, please say again." They didn't want to tell her what was happening, because they knew who was on the other end of the radio. This just adds to the mother's worry, as she can draw her own conclusions as to what's happening.
"Hayloft Hanging Hazard": a girl is playing on a rope swing in her family's barn. The swing has two loops as a handhold foothold. Unfortunately, the former is right at her head height, and as she's getting trying to get a hold on it, she loses her footing...
"Swarm Save" and "911 Honeybee Horror": Both featuring victims and rescue personnel being attacked and covered by thick swarms of bees. And the constant loud buzzing — Bee Afraid!
"Swarm Save" has a flatbed truck transporting beehives fall over on its side, agitating millions of bees. Imagine being not only injured and trapped by an automobile accident, but also swarmed by angry bees at night in the dark, and that the bees are so thick that any emergency personnel can't get to you or your passenger without being attacked themselves — one paramedic even goes into shock due to a previously unknown allergy.
"911 Honeybee Horror" tells the story of a man mowing some property who agitates a nest of feral honeybees. The responding deputy is allergic to bees, emergency responders can't get too close until some beekeepers arrive, and several of the bees latch onto the victim and get into the hospital and sting the doctors and nurses. But what makes the whole thing worse is watching the doctors pull bees out of the victim's nose...
And then there's the call itself:
"I'm being attacked by killer bees!"
"What is that, sir?"
"I'M BEING ATTACKED BY KILLER BEES!"
"911 Stalker Save," on multiple levels:
The victim's stalker is waiting for her after breaking into her house. If not for the cunning of the dispatchers and police, she would have been abducted at knifepoint.
She said the stalking had been going on since 1982. The segment is from 1990. She'd had to deal with this lunatic for eight years.
"Mother and Son Hide from Intruder": Pretty much the entire 911 call is incredibly frightening to listen to.
The mother is hiding in her closet and is hysterical, as she is well aware there is a dangerous man in the house who likely intends to do her harm. Worse is the fact that her gun is in a lockbox and the key is in another room. Worse than that is seeing the P.O.V. Cam walking to the closet, leading to...
Hearing screams on the recording as the burglar finds her, doubled with the show not depicting the ensuing scuffle (showing instead the dispatchers' and policemen's reactions). The dispatcher, Barbara, even mentions how helpless and chilling the whole thing is to listen to.
Barbara: I heard the whole conversation, and the thing that sent chills down my spine was when he said, "I'm here to solve your problems."
When the police arrive, the burglar attempts to use the young son as a human shield.
As he's being led away in handcuffs, the burglar gives the mother a Death Glare, which she later says feels like him silently swearing revenge.
After it's all done and over, Barbara hangs up the call and weeps, and the show points out that the mother and son have to undergo counseling.
An episode wherein a man cracks and shoots his wife five times and his daughter once (not intentional, but she was in the way). The call is nothing but incoherent screaming for the first couple minutes from all the kids who are in the kitchen while their dad is in the hallway outside even saying she is going to die. The man has absolutely no remorse, even calmly piping up in the police car to say he shot his wife five times.