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Nightmare Fuel: The Big Bang Theory
Greed transforms. Good thing it's a dream.
Howard's mother. We never see her, she always shouts, and according to Howard, he doesn't live with her, she lives with him.
Something about Leonard's mother's voice is a bit chilling. She sounds how a female version of AM would. Think of Leonard's childhood. She is really is comparable to the likes of SofiaLamb and DahliaGillespie. The way she Mind Rapes Penny with that smile on her face that's like Johan Liebert from Monster... forget suicide, it's a wonder Leonard didn't turn into a serial killer! (Sheldon actually once did muse on the possibility that Leonard is actually a violent sociopath, simply biding his time before actually killing him!)
The worst part is that its completely Played for Laughs, even when Leonard actually shows how much his mothers behavior upsets him. On top of that, his mother KNOWS that her behavior is abnormal and directly harmful to the development of her children, being an expert psychologist, but she has no interest whatsoever in changing.
TheMorlock scene. A dream within a dream, both with Morlocks that attack Sheldon no matter where he goes with the Time Machine.
Sheldon's fake smile with his eyes wide open and showing all of his teeth, to the point where it was the page image here for a long time. It's currently the page image for The Unsmile.
When Sheldon got even with Leonard for a scare prank. Not the prank itself, but the fact that the audience didn't see that coming.
Bernadette. Just Bernadette. At one moment, she would be cute, sweet, cuddley, the next moment she would use her scary voice to force you to do things.
Or let it slip that she accidentally combined Ebola with the common cold, or that she couldn't remember if she washed her hands after working with penicillin-resistant gonorrhea.
Sheldon's dream at the end of "The Santa Simulation". Sweet moment with Santa: Awww. Santa turns it around and shoots up the apartment with a cannon... Um, yeah, definitely a nightmare. Sheldon's face at the end when he wakes up shows the poor guy is obviously terrified and there's no ending joke to diffuse the tension for the emotionally confused audience. The audience that just saw Santa. Try. To. Kill. Sheldon.
The episode when Howard eats peanuts in order to keep Leonard away from his apartment for a surprise party. Howard's swollen face was TERRIFYING.
Sheldon gets 25 cats replace Amy in his life. The episode is hilarious and cute but this line is jarringly disturbing.
Leonard: Fine then, live with cats, be like my aunt Nancy! She had dozens of them and you know what happened after she died? They ate her!
In the episode where the guys fight over a prop of the One Ring, Sheldon has a nightmare where he gets it and becomesGollum. It isn't a pretty sight, and is currently the page image. Sweet dreams.
Literally, and in-show: for Penny (and Leonard), it's Howard and Leonard's explanation of how Sheldon might reproduce.
Howard: I'm an advocate of mitosis. One day he'll eat an enormous amount of Thai food and split into two Sheldons. Leonard: On the other hand, I believe he's the larvae form of his species and one day he'll split the cocoon and emerge with moth wings and an exoskeleton. Penny: Okay, well, thanks for the nightmares.
In Season 5, it's never shown what exactly was crawling around inside the couch Penny picked up off the street, and later picked up by Howard and Raj.
Given the characteristics of the bulge and its movement, probably a mouse or other small rodent. Still scary?
The face. Oh god, the face. Also the fact that this is still the closest we've come to seeing Howard's mother properly.
Not to mention Howard's creepy expression as he finishes telling this story. He's not just imagining, he's fantasizing.
Bernadette's fake laugh is one of the most terrifying moments in the series so far.
Early in the first season, Penny lets slip to Sheldon that Leonard may have made light of Sheldon's accomplishments. The sight of a genuinely pissed-off Sheldon, towering over Penny as she backs away, demanding to know what exactly Leonard said. Up to that point, Sheldon had largely been played as a Cloud Cuckoo Lander, but seeing that scene, you could very easily believe it when Leonard says that Sheldon is one lab accident away from becoming a super-villain.
When Howard is going into space the gang is sitting in the apartment watching the launch together. It's a heartwarming scene. It becomes Nighmare Fuel when you think about the Challenger disaster and consider what if it was your own friend/spouse in Howard's place.
Stuart's entire life. His misfortunes may seem funny to us who watch him, but try to put yourself in his shoes. His life is basically an endless series of nightmares that come true.