* The concept of video game characters coming to life sounds cool on paper, but as Mark finds out, his life is targeted by the psychotic villains, his loved ones are put in harm's way, and he could very much be harmed or even killed by the badguys. And anything can be summoned from the game world using the Amulet of Zoar.
* Chuck getting hit with an electrical blast by an emotionally disturbed Ace after accidentally firing Mark's wrist cannon at him. Chuck lies on the ground motionless and Ace freaks out until Mark arrives. Chuck awakens moments later, but it is still pretty shocking to the show's primary source of comic relief get hurt like that.
* [[spoiler: Sparx's DisneyDeath. She is electrocuted by her own sword wielded by Googler's puppets, and is sent back to the Sixth Dimension flickering and disintegrating. She comes back later on in good shape.]]
* If Ace doesn't power himself up with electricity, he starts flickering and becomes fuzzy like a bad TV station. It turns out if he doesn't charge himself, he will eventually fade into nothingness.
* Lord Fear is usually treated as a quirky Saturday morning villain, but he is a [[DemBones talking skeleton]] who often is accompanied by creepy cracking bone noises, particularly when he stretches his limbs. Oh, and he also plays the pipe organ, takes pleasure is scaring kids senseless, and has a wicked cackle.
* Lord Fear and Lady Illusion's...disturbing romantic relationship. We never see them kiss or anything apart from one embrace, but considering Lord Fear is an undead skeleton. Well, the rest can be left to your imagination.
* Random Virus is technically a good guy, but that doesn't stop his AxCrazy evil side from taking over to attack anyone he deems to weak or useless to live. His evil side could be viewed as a twisted version of his sense of justice, vowing to remove all weakness in the world - the only problem is that he thinks kindness and good virtues are evil.
** And the worst part is, we don't even know how he became a cyborg, other than he had some sort of [[NoodleIncident accident]].
* Googler the MonsterClown and his talking, zombie-creating sock puppets that can bite through any material with their teeth made of felt. In their debut, the puppets latch on Mark's arm, bite down, and Mark is rendered immediately unconscious. What did they do to him?
* Rotgut is perhaps the most AffablyEvil of the badguys, though mainly because he's so useless and ineffective as a villain, but that doesn't stop him from being grose when his limbs keep falling off. And then there's the time he [[DemonicPossession possesses Chuck]].
* Kilobyte is just downright creepy. His RedBaron name is the Cyber Stalker. His CombatTentacles have some {{Squick}} elements to them, able to absorb energy from other characters via his tentacles, or even upgrading them with new powers. He does this to Lady Illusion, [[DoesThisRemindYouOfAnything his tentacle entering her body to rewrite her code while she lets out gasps of awe.]]
** There's one point where Kilobyte rewards Lady Illusion for her loyalty to him by stroking her cheek with one of his tentacles.
** Kilobyte's change of character after he is freed from Rick's control. At first, he is a very cold, calculating with the mind of a hunter who wants to hunt Ace slowly and painfully. After being freed, he still has the basic personality but is now AxCrazy with a desire to take over the world. He becomes hellbent on trapping humanity in the game world, eliminating all loose ends in his plan like Mr. Cheseborough and Random, and laughs like a nutter. No wonder Ace and Lord Fear team up to defeat him.
** And let's not forget his giant wasp Fred, who was an ordinary bug until it stung Kilobyte, taking on his power and become a gigantic monstrosity.
* Episode 11 starts with Mark having a nightmare where he is lost in the carnival being chased by the villains, and Ace isn't there to save him. Lord Fear then starts stalking Mark around school and the streets in Duff's [[BadHumorTruck ice cream truck]]. Mark takes a wrong turn and the truck pulls up in front of him. Mark can only stare in terror as Lord Fear's head and arms poke out and he says "Come to, Papa!" before kidnapping him. Mark escapes at the carnival and his nightmare plays out in real life, with Lord Fear and Anvil cornering him. Mark lets out a SkywardScream, but this time, Ace does come to his rescue.
* Zombie Ace is pretty scary. Brainwashed by Googler's puppets into becoming a brainless servant with MindControlEyes, Ace [[ImplacableMan hunts Sparx relentlessly]] with the intention to kill her and then Mark.
* Brett, Heather, and background characters get trapped in the carnival funhouse by Lord Fear and there's no way out. The camerawork makes it look and feel very claustrophobic.
* Chuck's urban legend of the Radioactive Guy goes to near creepypasta levels. The story involves an astronomer named Cornelius Fowler (portrayed by Mr. Cheseborough) who spots a radioactive comet crash on Earth. He chisels the remains apart, only for it to suddenly break in two and expose him to radiation. Chuck then reveals Fowler went nuts and sealed himself in the observatory, and his ghost haunts the building. We then cut to Fowler, with crazy white hair and a psychotic expression barricading himself in the astronomer. A man outside shouts at him to come out for help, and we get this creepy chat:
-->'''Man Outside:''' "Cornelius, we just want to help you."
-->'''Fowler:''' "You can't come in. No one can ever come in again! ''I'm radioactive!'' I'm radioactive! [[MadnessMantra I'M RADIOACTIVE!]]"
* Mr. Cheseborough's predicament throughout the series. He is kidnapped by Lord Fear and replaced by Lady Illusion in a plan to destroy Mark, and kept in a cage at the carnival. Lord Fear frees him but has Dirty Rat erase his memory of the experience. Turns out that only partially worked. Mr. Cheseborough remembers the whole things, thinking he was abducted by aliens, and slowly goes mad over the course of the series with everyone thinking he is a looney. This costs Mr. Cheseborough his job at the elementary school, and by the end of the series when he is trapped in the game world, he has gone quite frantic and is condemned as a wanted man by the police.
* Duff's puppet Huckleberry goes into UncannyValley territory.
* The Carnival of Doom is quite a creepy place. While it avoids the AmusementParkOfDoom traditions, it still has a lot of disturbing noises like creaking rides and spooky gusts of wind. Not to mention it is home to a bunch of supervillains who could jump around the corner to capture any unsuspecting intruders as Mark, Chuck, Sparx, and Kat have experienced.
* Pigface is a fast-moving PigMan able to devour anything and has JabbaTableManners. Now imagine that thing chasing you.
* A moment from the first episode where Mark hears noises on his roof and looks out the window, only for Lord Fear's head to appear upside-down.
* Kilobyte's debut is creepy. Emerging from the carnival's ferris wheel in the dead of night, and we never see his face for the whole episode. Only watching the other characters while making eerie statements about his plans for them.
* Lord Fear and Staff Head's return in the second season. Wayne goes looking for them in the carnival at night. Going to the haunted house, a ghost train mysteriously slides out to meet him and Wayne goes around the ride looking for Lord Fear. In the back of the cart are two fake skeletons that start glowing. When Wayne exits the ride, Lord Fear and Staff Head have replaced the skeletons and sitting behind Wayne. They then scare the pants off of him.
* Ace's story arc in the second season. He is infected with Lady Illusion's EmotionBomb, erasing his original programming and leaving him with out of control emotions that effect his actions - he spends one episode really angry and taking his rage out on Lord Fear, Random, and Chuck. The next he encounters Kilobyte and is terrified out of his mind to the point he has an HeroicBSOD and thinks there is no reason to being a hero anymore until Mark proves him wrong.
* Lady Illusion has her moments. Aside from her {{Yandere}} qualities around Ace, she can shapeshift into any person or animal. ParanoiaFuel indeed. Then there's the episode she shapeshifts into Mark and tries to destroy his social life while the real one is in bed with a cold. Imagine that happening in real life.
* The episode "The Not So Great Outdoors" has Mark wandering in the dark forest with creepy spotlights accompanying him - then he runs into Staff Head, Pigface, and Dirty Rat who gang up on him to kill him. He deceives them with Chuck's portable version of the game.
* We only see glimpses of the Sixth Dimension, but it looks like a dark CrapsackWorld with EverythingTryingToKillYou. You get a more better look in the PC and Playstation 2 games, but it is still a nightmarish hellhole where everything is a monster trying to kill you.
* The Playstation 2 game's version of the carnival is dark, empty, and quiet. The haunted house has giant animatronic heads of Lord Fear and Staff Head, staring out over their domain. The Gameboy Advance's version looks quite ordinary aside that the villains can jump out of nowhere to issue a sudden boss fight.
* The game's soundtrack can be very unnerving and creepy.
* In the Gameboy version, the first level is a ghost train where you encounter a vampire who jumps out of a coffin. The player has to dispatch the vampire immediately with an electric blast or he will follow the player throughout the level, even if you use the flying power up to try to move on to avoid him, he flies after you!
* The Sixth Dimension comes with a CircusOfFear, Lady Illusion's terrifying manorhouse which is nicknamed the "Freaky Funhouse" right out of a DisneyAcidSequence, the WeirdWest where everything is undead, and a medieval castle built atop a volcano.
* Among the monsters of the Sixth Dimension are - acrobats whose bodies have been twisted back-to-front, killer robotic clowns, [[OurGnomesAreWeirder evil garden gnomes]] whom Ace despises in the series, zombies riding mowing machines made from ''bones'', [[ManEatingPlant giant flytraps]], killer [[AnimateInanimateObject wardrobes, ovens, and fridges]], maids, [[BattleButler butlers]], and chefs who have been bitten by the infestation of spiders and turned into mutant freaks, the ''giant'' spiders in Lady Illusion's boss fight, the MadBomber canaries who are strapped to time bombs and explode in your face, zombies dressed in cactus suits, CirclingVultures, [[{{Hellhound}} Hellhounds]], strange freaky one-eyed chicks, and weapon-wielding ferrets. Heck, even the tumbleweeds attack you!