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Nightmare Fuel: House
The entire series premise in general where a person is suffering and or dying from something that cannot be explained or cured by a regular doctor so a certified genius has to be brought in who then goes through series of wrong diagnoses before finally coming up with the right solution at the very last minute!
House throws in some more-or-less Real LifeSquickfest whenever possible. The Season 2 finale "No Reason" features a man whose body organs swell up and sometimes explode, including his tongue, testicles and an eyeball. It all turns out to be an hallucination on Dr. House's part.
Perhaps the worst moment comes at the end, when House takes control of the robot surgeon and slices the man open from stomach to chest with the intent of killing him so the hallucination will end. This takes just long enough to make you wonder if maybe he was wrong about everything being a hallucination, and he might have just actually killed a man. Alternately, of course, House might have been putting the man out of his misery. In a very, very VERY loose sense of the word.
Remember "Not Cancer", the Season 5 episode with the four dead transplant patients from five years ago, where the fifth hallucinated, without segue or warning, that House (otherwise perfectly in character) was performing an impromptu capitoectomy with a meat cleaver? Watch one of your favorite characters, one who occasionally carries out seemingly irrational medical procedures, attempt to remove a patient's head without batting an eye.
The death montage that started that episode was itself extremely disturbing.
Similarly, in the season 1 episode Paternity, House walks into a patient's room, casually ties him down and then amputates his toe with bolt cutters (it's just another hallucination).
The season 4 episode "Guardian Angels", involving a woman who worked in a funeral home who kept having hallucinations. As well as seeing her dead mother, House's grandfather and the patient Thirteen accidentally killed last episode, there was a very creepy scene where Kutner stabbed the patient's arm with a hypodermic while Thirteen held her down, and then Kutner pinned her down and Thirteen stabbed the needle into her arm, both of them looking at each other with really creepy smiles and then smiling at all the blood. Then the patient wakes up, covered in sweat, to see Thirteen leaning over her, trying to reassure her that she's OK and it was a nightmare. The patient says that her arm is bleeding; Thirteen thinks it's just a part of the nightmare... until she looks at the patient's arm... and there are the stab wounds, plus the blood.
In season 3's "Insensitive", a girl who was physically incapable of feeling pain was brought into surgery without anesthesia. House cuts her stomach open while she's watching, producing a 25-ft tapeworm that he proceeds to extract before her eyes.
The Season 3 episode, "Resignation". The patient's in the MRI, and she starts complaining that her head hurts. Her scalp is split open. Her illness caused her stomach acid to flow through her vascular system, collect at the top of her skull AND BURN THROUGH! Foreman later mentions it looks like "Acid tissue death".
Another Season 3 example, "Family". The scenario: two young black brothers are willing to risk their own lives to cure the other. One needs bone marrow for the entire episode, but the other brother gets an infection that jeopardizes any chance of a bone marrow infusion. Eventually, the donor brother gets cured of his infection, but Foreman convinced him to do an emergency bone marrow extraction without anesthetic to save his brother. Forcefully and swiftly. The screaming of that kid never leaves your head, and that was after filling only two syringes with bone marrow: Foreman implies that he needs a lot more. Shudder. Between torturing the kid to cure his brother and accidentally killing a patient the previous episode, no wonder Foreman wanted to get away from House and his Despair Event Horizon inducing practices.
None of these hold a candle to the Season 5 endgame starting from the moment Kutner kills himself. The penultimate episode of the season turned House's increase in mental instabilityUp to Eleven. Enjoy yourself, it's later than you think...
Absolutely anything featuring the hallucination of Amber's ghost. Especially the terrifying ghostly rendition of Enjoy Yourself... arghhh!
From a more emotional standpoint, there's something so very wrong about House having to stay in a mental institution. That last shot of him looking terrified is creepy.
In "Who's Your Daddy?", the penultimate episode of Season 2, House has to prove to his team that the patient's hallucinations are pain induced. He pricks her with a needle, and when that doesn't give results, he breaks her finger. Cue a point-of-view shot from the patient's eyes and the audience getting to see Hugh Laurie's face melt. Lovely.
In the Season 5 episode "Joy", Taub tells a patient: "I think... you're sweating blood." Made all the more awful because even Taub sounds utterly creeped out.
The psychopath in Season 6's "Remorse". That creepy glance she shot at Dr. Hadley was priceless.
"Black Hole". The visions the patient has of falling into a black hole. * shivers*
Greatly reduced horror in the next scene when the following exchange takes place:
Foreman's face is in frame. Switch frame to the patient. The patient: It's a black hole. Foreman: She's hallucinating. Taub: I certainly hope so.
"Forever" from Season 2 has a father leaving his wife and new baby to go to work even though he is sick. At the door he decides he is too sick to go so returns upstairs, where he finds his wife having some sort of seizure in the tub. That's scary enough. Then he turns and sees his newborn baby in the bottom of the tub. To any parent that is more horrifying than drowning yourself. Towards the end of the episode, we see the mother smothering her own child
"Babies and Bathwater" in season one forced a to choose between saving his wife and his unborn child. He chose his wife, but then complications ensued and she started to bleed to death with no way to save her. The husband then had to give the doctors permission to actively kill his wife by removing the baby or else both mother and baby would die - the only decision, really, but still incredibly difficult to have to go through with.
House and Chase have both performed autopsies on babies.
Season 7's "Massage Therapy" features a schizophrenia-induced hallucination that looks to be straight out of a horror movie, with all sorts of Body Horror. House finally convinces the patient that she's hallucinating by calmly placing his hand on a table that she informs him is on fire, which it naturally appears to be to the audience until House dispels the illusion.
In Season 7's "The Last Temptation", the patient, who postpones a surgery in order to complete a month-long sailing race, is drugged into a cardiac event. She later wakes up without her arm.
Season 7's "After Hours". House performing surgery on his own leg with the intent on cutting out tumors was bad enough, but then his hand had to start shaking and the scalpel slipped and the blood and the shaky cam...
Season 1's "Detox", especially the part where the withdrawal starts taking its toll on House, and he breaks his fingers with a pestle!
The hallucinations that the boy has in Cane and Able.
Another particularly horrible example is found in the Season 2 two-parter "Euphoria" and the fate of the patient which as it turns out has spread to Foreman. Having a disease that constantly sets off all your pain receptors without as much as a finger prick has got to be one of the worst things imaginable, much less having it happen to a friggin' Main Character.
When House jumps off the balcony and into the swimming pool in "Out of the Chute" (Season 7) it appears to viewers and the onlooking Wilson that he's going to commit suicide. His behaviour is so manic and impenetrable that it might really have been one of his options in the wake of his breakup with Cuddy.
Wilson's chemo in "The C Word". The sweating, shaking, vomiting, hallucinations, him almost going insane...
Season 1's "Maternity". The entire episode, really, but especially the newborn lying on the far too large autopsy table. Nightmare Fuel mixed with Adult Fear at it's most unsettling.
waking up Amber in "Wilson's Heart". Wilson seemed horrified at the idea, but everyone seemed to think it was perfectly okay to wake up a dying woman just to tell her she was dying and have a bunch of people she didn't even like say their goodbyes.
House's hallucinations of Amber quickly veer from amusing/annoying right into this trope. That scene where she's singing at the restaurant? Creeepy.