The episode "Fancy Pants". In order to get into Peppermint Larry's VIP section of the Candy Barrel, Captain K'nuckles has to get a pair of fancy pants. But since K'nuckles is...well, K'nuckles (i.e. dirt poor), he can't buy a pair. However, there is a free pair, but you have to be incredibly obese to wear them. While trying to figure out a way to gain enough weight, Flapjack suggests to K'nuckles that he should eat a ton instantly fattening "candy-coated hot dogs". Seeing K'nuckles eat the food is bad enough, but what comes at the end of episode is even more horrible and completely out of left field. K'nuckles finally gets into the V.I.P. section, but all of a sudden he explodes, filling the place with a billion hotdogs and a sea of BLOOD.
The cat which provides our lovely page image. "Eeeeeeuuuuhhhhh..."
In "Foot Burn" an old hag shouts at Knuckles causing his face to rip off. The results are very detailed.
The hag herself is also pretty disturbing.
Almost all of " K'nuckles Is a Dirty Rat" is nightmare fuel. What with all the extremely detailed, extremely grotesque, plague infested citizens, Dr. Barber, and the random song about giving someone the plague just because they won't be friends with you.
"Pun Times with Punsy McHale" has a scene where he scares Flapjack by telling him puns as his head enlarges and turns to all the colors of the rainbow. "You think you're RED-y for my rhymes!?"
The acid trip like sequence K'nuckles has in "K'nuckles' Hilarious Problem".
The character Dr. Barber. Here's an explanation: being in a show that is Courage the Cowardly Dog combined with SpongeBob and The Ren & Stimpy Show, he's one of the scariest characters there. Plus, he's involved in most of the scary scenes of the show— like the freak-out scene from K'nuckles and His Hilarious Problem,Fish Heads, and K'nuckles is a Dirty Rat.
The episode "Fish Heads" has the K'nuckles and Flapjack selling hundreds of fishheads for various purposes such as toothpaste. Then, when Doctor Barber gets a hold of the two's fish heads, he creates a giant hair...fish...thing which was supposed to give everyone perfect haircuts, but instead gave them horrible haircuts. That's not all. It's a lot scarier in action.
Gone Wishin' has the mermaid which decays over time from K'nuckles wishing, and then her horrifying transformation sequence.
Flapjack's nightmare from "Tee Hee Tummy Tums". What is scary about it is that it comes straight out of nowhere.
in Panfake Peppermint Larry uses DEAD RATS as puppets! While Flapjack and K'nuckles are helping Peppermint Larry by cleaning out the Candy Barrel, Flapajck stumbles upon an old box, Larry remembers that the box has his old puppets in them so he pulls them out to show him. When he pulls them out, Flapjack just flops to the floor screaming in terror. From a distance they look like rubber cartoon rat puppets, then Larry has to use them to point out they're puppets and we clearly see they're dead rats tied to strings! "shudders" no WONDER the Dock hag shut him down!
A subtle but terrifying detail is that at first Larry voices each puppet one at a time. Then he somehow voices both puppets simultaneously. Either he is the greatest ventriloquist of all time or the puppets can really speak.
That mechanical genie...
The sight of Flapjack cheerily tying knots in Bubbie's intestines in preparation for the Knot Tying Festival. The fact that everyone treats it so lightly just adds to the Mind Screw factor.