In "Free Cake" when Skips is sentenced by the Guardians of Youth to shrivel up into dust and disappear forever. Cue Skips rapidly aging, then collapsing to the ground slowly dying, his skin turning into dust while his skeleton stays intact. He even tries to speak, but can't, only groan in a whisperly voice. But he gets better once Mordecai and Rigby make an Equivalent Exchange by sacrificing the cake.
In both senses, meaning they intend to eat the guys but end up eating themselves. Actually, each of them eat the other while being eaten themselves. The leader is the only who shocked by this, but can do nothing as mustard fell on him and he gets eaten as well.
The climax in "Grilled Cheese Deluxe" when Mordecai and Rigby end up preventing the anti-matter from destroying the city by sacrificing the grilled cheese sandwich only to be zapped by the anti-matter and get distorted and stretched in freaky ways. At one point, Rigby gets turned inside out. Mordecai got stretched out to the point where he was barely visible. And let's not forget about Rigby being ripped in half and being put back together in a matter of seconds.
The demonic taxi that haunted Rigby's nightmares in "Ello Gov'nor" was actually pretty frightening at times, especially during one of Rigby's dreams where it crashes through the window, baring its teeth and screeching, accompanied by the freakiest Scare Chord ever. You can't blame Rigby for getting so freaked out by that thing.
When Mordecai sends the microwave barreling toward the end of time with him and Rigby on it in "It's Time", during their argument Mordecai pushes Rigby off of the microwave and he disintegrates into nothing but a skeleton with eyes before shriveling into a bunch of dust. Then the Fridge Horror kicks in when you realize that Mordecai will be haunted the memory of him killing his best friend in an alternate timeline for the rest of his life...
Snowballs the Ice Monster is a giant serpentine creature with rubies for eyes, a huge lamprey-like mouth, and can, for some reason, breathe fire.
And when Mordecai beats him, he's stared so hard that he can't close his eyes anymore. When someone tries to put drops in his eyes, they evaporate to steam before hitting his eyeballs.
Mindless Mordecai in "Brain Eraser."
"ALL DAY, EVERY DAY."
The video they were watching was kinda cute, though.
Susan, the evil boss in "Benson Be Gone". Her head turned 180 degrees and floated out of the house screaming before going One-Winged Angel and Ax Crazy.
Her, and the fact everybody except Benson start to transform into her with their faces still intact because of her horrible influence. Even worse, if doing what Susan tells you to do turns you into Susan, then who's to say that the Susan hired by the park owner isn't some poor sap who was completely transformed into Susan? And when Benson gets back, Pops, Skips, Muscle Man and High-Five Ghost are almost all totally taken over by Susan; much longer and they would've been totally gone.
The were-skunk was actually an innocent human that let the effects of another were-skunk get the best of him. That means that the true were-skunk is still out there, along with all of the people and animals that have become fully transformed into were-skunks.
The Season 3 premier episode "Stick Hockey" has a flashback where Benson and his student, who looks exactly like him but is orange, are in the stick hockey championships. Benson wins his round, but the student gets his head decapitated by the game sticks. Benson picks up the head of his student as he watches his student's final moments slip away. He then says, "Master, I thought you said that this was going to be so much fun!". Immediately afterwards, he dies.
Muscle Man being flayed alive, Mordecai being decapitated, Skips being incinerated, and High Five being shredded. The Halloween episode is pretty grim.
There's also that scene from the end of "Death Metal Crash Pit" where the RV crushes the ghost band, causing an explosion that skins, burns, and melts the entire audience, even showing eyeballs hanging out of their skulls! It even kills Muscle Man and Hi-Five Ghost.
The freak deer in "Camping Can be Cool." And the implication that he had a human mom.
Mordecai and Rigby's willingness to disguise Benson's head injury in "Weekend at Benson's." In real life, the recipient of that kind of injury would need to be taken to a hospital immediately to be checked out for brain trauma.
In "House Rules", learning that your best friend, who you grew up with since you were kids and care for like a brother, may end up dying young and you don't know where, when or how and you can't stop it and you can't tell anyone for fear it will slip to your best friend so you're trapped with this horrible secret.
"Think Positive". Benson is the source for this episode's supernatural happening, of note as even in episodes where main characters cause the trouble, they're never the trouble themselves. Basically, Benson has to keep his Hair-Trigger Temper under control if he doesn't want to get fired by Pops. After enduring two humongous screw-ups by Mordecai and Rigby, he hits the wall and wrecks his own house to let some of his anger out. The very next day, Mordecai and Rigby make Benson so pissed off that he literally goes Super Nova and the only way for Benson not to kill himself from holding in his anger is for Pops to allow Benson to yell at them again. And Benson does this to the point where he's in tears from the Unstoppable Rage and Mordecai and Rigby go deaf!
Benson's family. No wonder he's so mean... Especially since families like that actually do exist in Real Life. Sometimes they are even worse than Benson's.
The virus creature, Doomageddon, in "Skips vs. Technology". It possesses Techmo, speaks in a creepy voice, and digitizes anything it touches.
In "Death at Eight" Death makes Mordecai and Rigby babysit his son to win back Muscle Man's soul after an accident. While they do manage to do it well, it gets creepy when Death mentions that they "did a better job than the last few babysitters". He then lifts his hand and reveals that he is wearing three rings with tortured souls inside. Yikes.
"Trucker Hall of Fame." The Truckers serving as the Big Bads of the episode. Not only do they emotionally bully Muscle Man every chance they get, they try to kill Muscle Man, Mordecai, and Rigby with their truck in the end and would've succeeded if the ghost of Muscle Man's dad didn't take them out. The scary part is they're not monsters, they're normal people who hold a grudge on his dad and are willing to kill him for it!
"Diary" had Skips perform a ritual right out of Raiders of the Lost Ark with groaning spirits, and when Rigby messes it up the spirits form into a giant close-up of Margaret's face, twisted into look of pure hatred and gives a bone-chilling shriek.
ButtonWillow McButtonWillow from "The Best VHS in the World" was creepy as fuck- thanks to his obsessiveness, self-seclusion, and... well, nuttiness.
Although he is hilarious if you know Gollum from Lord Of The Rings, who he is a parody of (although he looks like a stereotypical fantasy elf or a redcap).
The scary music in "Death Bear", along with the abandoned zoo imagery, was very unsettling.
The fact that all those tranquilizers wouldn't subdue Death Bear until Mordecai freaking filled him full of them at point blank range.
The new Halloween Episode. The commercial for it involves Mordecai's uncle meeting his demise inside of one of those things that return bowling balls to you. Somehow made worse by showing a clip of a tree in a woodchipper instead of the actual death.
Mordecai's story. In the story, he sees his Uncle Steve die by falling into the thing mentioned in the above paragraph. He has nightmares from the stress, and sees his uncle, (complete with spooky, ghostly eyes and a blank stare) around the house, in the paintings, in the mirrors. Eventually, Steve's ghost chases down Mordecai, finally cornering him near a grave. It turns out that Uncle Steve had "unfinished business," and simply wanted to return five dollars to Mordecai, before returning to his resting place.
Margaret has a pretty fucked-up sense of humor when it came to her ghost story; the story involves Mordecai, Rigby, Margaret, and Eileen needing a ride to a movie theater since Benson won't let them use the cart on account of pranksters TP'ing the park. Eileen calls a cab but a strange bus called "The Party Bus" pulls over in front of them before the cab showed up, they then board the bus and are greeted by a bus driver in mummy costume and join the other party-goers on board. While they were having a good time as they waited to arrived to the theaters, they soon found out the party-goers were aging to dust, and so were they. Mordecai and the others desperately tried to escape the bus before they die, but the driver, who turned out to be a skeleton, persistently prevented them from getting off the bus. Mordecai managed to reverse the bus' aging effect on them by shifting the bus in reverse, but it workedtoo well as the reversed aging effect made them so young they reverted to being children. They finally escaped through the bus' escape hatch, which at this point they've became mere infants, and jumped off the psycho bus. Unfortunately it was too little too lateas they all de-age to children to the point that they disappear, and this horrifying tale ends with the bus driver laughing menacingly as the bus fades way to nothingness. And that was suppose to be funny? Even Rigby found that story pretty fucked-up.
Benson's story is also unsettling, especially if you're claustrophobic and/or arachnophobic It starts off unassuming, with Benson ordering Mordecai and Rigby to re-wallpaper the entire house because of their vandalism and the current wallpaper being discontinued. They complained about not wanting to do it, only for a television to appear in the hallway (which even Muscle Man is puzzled by) showing a man named "Jan, the Wallpaper Man" offering to wallpaper the home for them for free. Jan finally arrived to get started while Mordecai and Rigby run up stairs to play games; Jan was acting suspicious the whole time. After playing games for a few hours, Mordecai and Rigby were amazed by the wallpaper, which had a peculiar color. Just as they were about to go downstairs, they heard Pops screaming and found him bound to the wall. After freeing Pops, they hear Benson screaming and traverse a confusing maze of wallpaper and chase after Jan climbing on the ceiling until they get trapped in an enclosed space. Rigby panics and eventually tears down the wall. They tear down more wallpaper until they finally reach a cave where the other employees were tangled up. They find Jan, who turns into a giant spider. He eats Muscle Man alive, and when he tries to eat Mordecai and Rigby, he ends up eating a box of grenades instead. The explosion killed Mordecai, Rigby, Jan, and Muscle Man (if being eaten didn't kill him first), and one of their intestines lands on Pops' head.
The original ending to the episode. Not that the version that aired on TV (where the driver crashes into the tree and all of them come out as ghosts) was any better...this one cranks it up to the MAX. After Benson finishes his story, the driver drives over to let the gang see that there are dead clones of themselves being covered up by a skeleton-faced paramedic. The driver turns out to be a skeleton too, and he drives the screaming-in-fear workers into a fiery vortex in which their spirits fly out of. This all happens inside a pumpkin that says "HAPPY HALLOWEEN!!".
In "Guy's Night", those little white creatures. When they remove their mask they reveal that they only have a large mouth with sharp teeth and getting bit by them will turn you into one of them.
"That's My Television." RGB 2 was apparently a LITERAL slave to the television executives. He could only survive by breathing air from the 80s (it was canned.) Everyone was trying to KILL him for trying to make a run for it. And the most horrifying part is that RGB 2 wasn't an anthropomorphic television- he was an actual emaciated, sickly HUMAN BEING inside of a television! The end of the episode also strongly implied that he died shortly after the events we saw.
The leader of the Wickets creepily chanting in "Picking Up Margret."
Leader: Mordecai, put your phone away!
This is a parody of a scene from the movie, 'The Warriors'. Luther: Warriors, come out to plaaay!
Skip's Stess. Oh God, Skip's Stress. Where to start? First off, skips is feeling rushed to do too many jobs, then he finds this black fire mark on his arm, then he's told if he doesn't stop working, he'll die. Skips begins to take a nap then has a nightmarish dream where Benson gets eaten be a shark, Pops turns into the hellish black flame with a distorted voice, the creepy close up on Muscle Man's face...◊ Then there's the close up of Skip's heart getting covered in the black flame◊. This episode is probably the most nightmarish episode to date.