Regular Show/Nightmare Fuel

Of course this is a children's show, how could it not be?

It's just a Regular Show, right? Well, in actuality, it's anything but.

Spoilers abound below.

Muscleman being flayed alive, Mordecai being decapitated, Skips being incinerated, and High Five being shredded. The Halloween episode is pretty grim.

Muscleman: I told you I was ripped.

- Percy was friggin creepy as well.
The wizard's face in the candy. Some would argue the wizard himself.
- The finale, in which Rigby tears off his skin and is actually the wizard. And in addition to that, what happened to the real Rigby?

"You're all gonna pay..."

- And they all pay do in horrifically graphic ways, including a decapitation.
- The scene where Mordecai, Rigby and Pops try to find Percy in the creep-tastic basement. The parody of the Halloween theme song doesn't help.
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-Fives! (though the latter was already a ghost to begin with). Luckily, it was just a "scary" story being told by Muscle Man and Hi-Fives.
- Muscle Man's screams of terror, as he's trapped in the RV, desperately trying to get out. Usually his pig squeal sounds hilarious, but it's really genuine here...
The creepy pedophile-esque guy from "A Bunch of Baby Ducks."
The evil demonic Rigby in the mirror in Jinx. Its really huge and turns whoever it kills into an evil demonic version of themself.
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.
Rigby in the hospital, in "Egg-celent."
Iacedrom and Ybgir. Those two head straight into the Uncanny Valley.
Muscle Man's freakout during "My Mom." Up until The Reveal, he seemed completely psycho!
Skips killing Rigby in arm-wrestling.
Leon dying in "Benson Be Gone."
What the unicorns do to Benson in "The Unicorns Have Got To Go" looks a lot like rape... Not surprising, since one of the unicorns is an Expy of Alex of A Clockwork Orange fame.
The implication that all of those people, stuck in infinite loops in "Viral Video," were killed.
The Uncanny Valley blondes. They all look nearly exactly alike, and are part of a secret cult that has an initiation where you must lower a "normal-hair" into a pit of boiling oil.
Starla, shes the definition of Fan Disservice, and if you make her mad, she'll cause enough damage to make a twister look like a gust of wind.
Benson repeatedly getting beaten up and mugged in the fortune cookie episode. Hilarious to some people, but to people who've been randomly assaulted, it's just disturbing.
Benson's family in "Think Positive." 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 "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.
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.
The hot dogs Rigby gets from the freezer in "Meat Your Maker" are alive. And they're cannibals.
- 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.
Peeps is giant eyeball that will always be watching you, everywhere you go, until you die. Mordecai challenges him to a staring contest and if he wins, Peeps must leave, but if Peeps wins, he gets their eyes... and he will do so using an ice pick and an ice cream scoop. Oh and he has a rather creepy voice and can grow multiple tentacle-like eyeballs.
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 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.
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.
"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!
"Don" has the audit itself. Yes, everyone gets saved by The Power of Love in the end, but still: had that not happened, the entire park would have probably ceased to exist.