The Cy-Bugs in general. How they can adapt to just about anything simply from eating things. Anything. And these guys eat a lot of things. Every time they eat something, start visualising it merging.
The merging aspect is perhaps the most horrific part of their biology for the characters who have to deal with them , especially if they eat a sentient being. Nightmare Fuel, And I Must Scream, and Tearjerker all in one.
The way that they swarm and crawl around once they've invaded Sugar Rush is just nasty. If you're an entomophobe, expect to cover your eyes a few times during this film.
All those THOUSANDS of Cybugs you saw in Sugar Rush were bred by one.
So, if you die in another game, you're GONE. Your home game will suddenly be without a major character. If that character is the hero or the villain, center of the entire game, that's a quick ticket to OUT-OF-ORDER for your entire home game. Ralph's wanderlust could have doomed his entire home game, especially if he had managed to get himself killed.
The whole flashback as Felix describe Turbo's story, especially at the end when the arcades are wheeled out. The angle and music make it look like a funeral march. And considering these arcades are virtually individual worlds with sentient inhabitants, that's not too far from the truth.
What happens to ''actual'' glitches in arcade games? Are they all doomed to live lives of isolation in Dummied Out levels as long as their game is plugged in? And worse, not have anyone to call out to because glitches cannot leave their home game?
Markowski, the soldier from Hero's Duty. After only a week of being plugged in, he's completely shell shocked and petrified out of his wits by the thought of fighting Cy-Bugs. He's walking into walls and reciting a Madness Mantra, the sight of a tiny insect enough to make him scream in terror. A normal playthrough is clearly hell on Earth for him — and yet that's exactly what he'll be forced to do, over and over and over, until the day their game is unplugged.
It's implied that he's programmed to act this way, but if anything, it makes it even worse, since it means he has no choice at all with how he acts.
Technically, it's worse for Calhoun — Markowski could presumably take the occasional day off for a therapy session and no one would notice. But Calhoun is a major game character guiding the player — no matter how scared, hurt, or emotionally compromised she is, she has to be there.
Some additional Fridge Horror: Remember the scene where Ralph throws his medal at the screen and dislodges the "out of order" sign? That sign isn't part of the game. Video game characters and objects can interact with real, physical objects. Man, it's a good thing the game world doesn't include any murderous psychopaths or mindlessly destructive monsters- OH WAIT.
Let's make this worse. At the beginning of the game, Ralph casually mentions "All those guys from Asteroids - who knows where those guys are." Did they not get out in time?
In real arcades, machines are moved about and cords are tripped on all the time. Also most arcades turn things off during the night. Also, what if a blackout occurs. Most games wouldn't get any kind of warning.
If Litwak's has wifi, they could theoretically all evacuate into the Internet.
Which makes it even scarier if the Cybugs weren't stopped. As Calhoun mentioned, they would continue to infect more and more places granted they have a connection to do so.
It's likely Calhoun would blow up the exit to Hero's Duty in the case of an expected unplugging. Unless you just meant cy-bugs escaping in general.
There was once a murderous madsprite that managed to effect the real world before. Evil Otto killed two people. Why he stopped is anyone's guess, because he sure didn't change his ways.
More Fridge Horror for you. King Candy's explanation about what will happen to glitches if the plug is pulled easily explains why with PC games some glitches can be fixed simply by reinstalling the game. Non-glitched characters can presumably escape and seek refuge somewhere until the game is reinstalled, at which point they return, which is why they are still present when the game is installed again. But the reason the glitches are gone is because in the process of uninstalling you killed them horribly!
A positive note: Don't forget patches. Which heal major game problems without needing to uninstall, as well as add new content. Just imagine if Sugar Rush was for the PC, and Vanellope wakes up one morning to find out she's not only not glitched anymore but has new racetracks and new racers.
In this movie's reality, glitches cannot leave their game nor can they be killed within their own game making them immortal practically. Want some pure fridge horror? Think about real video game glitches, some can be terrifying as hell, now imagine being trapped with those glitches for the rest of your life.
Turbo. His attitude, his demeanor, and his physical appearance. He's responsible for destroying two entire worlds, and all because of his ego.
The scene where he is attacking Vanellope, and his true form is revealed. All the closeups of his glitching, screaming face (with pale skin and yellow teeth) are quite intense, and very jarring compared to what King Candy had been like up to this point.
...And right after that line, he flips her car over and tries to ram the poor little girl headfirst into a stalagmite dividing a fork in the road. If you hated smashing into those things then, you really gotta hate them now.
Imagine the scenario from Ralph and Felix's perspective. All their lives, Turbo was the closest thing they ever had to a real criminal in their arcade. Now, imagine that after thinking all this time that the worst man in your existence was gone for good, only to see him suddenly reappear in front of one of the few programs who did anything good for you (In Ralph's case). Is it any wonder Ralph's reaction is "NO WAY!!"
Turbo's appearance as the King Cybug, and his death, where he is incinerated. We get one last look at Turbo's face, as he screams in fear.
The fact that he knows he is flying straight into the boiling Coke geyser and is helpless to stop himself from doing so.
When Turbo finally dies notice how he does not stop screaming until he is completely gone.
"Let's watch her die together, shall we?" This is probably the most nightmare-inducing line in the entire movie. Think about it: Ralph has known Vanellope for not even two days, and she's like a little sister to him. And now Ralph is being held hundreds of feet in the air by a monstrous King Candy/Cybug hybrid and forced to watch one of the only real friends he's ever had get eaten alive by an army of Cybugs.
"I should thank you.......Buuuuut it'd be more fun to kill you!".
If, as theorized above, virtual objects can interact with real objects, then what would have happened if the Cy-Bugs were not stopped? Once they'd wiped all the arcade games clean, they probably would have broken out of the virtual world and started attacking and infecting gamers in the real world! A truly terrifying thought.
These twolovely freeze frame bonuses of Turbo's reveal. Sweet dreams!
To add to it, Turbo even flashes his signature thumbs-up (along with a creepy fluorescent-yellow grin) at the camera for a very brief second. No, really  ◊
Some more Fridge Horror: If characters always come back to life in their own games and Turbo programmed himself into Sugar Rush....
King Candy's Imagine Spot of Sugar Rush being unplugged. Because Vanellope's a glitch, she wouldn't be able to escape with the rest of the characters, and he imagines her pounding on the barrier keeping her in the game, screaming in terror as Sugar Rush collapses around her.
Not to mention that exact scenario almost happened scene for scene (Too close for comfort)
Calhoun's traumatic flashbacks of her previous fiancee, triggered by Felix calling her "one dynamite gal", can resonate too close for comfort for those who have some sort of emotional trigger.
Even more Fridge Horror for you all. After the Cybug ate King Candy/Turbo, he fused with it, right? So imagine what Calhoun might have been shooting at after her fiancee was eaten during the wedding.
There are literally dozens of Wreck-It Ralph askblogs/roleplay blogs on Tumblr. Most of these are relatively harmless and kid-friendly. However, there are a few out there that are pretty creepy.
fixit-felixbug. Basically, an AU (alternate universe) version of Fix-It Felix, made into a gigantic, bloodthirsty Cybug/human hybrid. It's a pretty big break from the cute little handyman we all know and love from the film.
creeper-pasta Simply put, a post-movie Turbo who "imperfectly regenerated after burning to death in the “beacon.” His glitchy code was further corrupted by Sugar Rush resetting while he was still in it. He now stalks the internet as a code-hungry, power-tripping, creepypasta nightmare virus... " Don't let his stature fool you - his other forms aren't as safe - and certainly not the type you'd like to come face-to-face with.