Ice King's giant pile of boogers in "Holly Jolly Secrets". Just... eugh.
Projectile vomiting into people's faces. There were two instances - Jake puking in Finn's face in "Storytelling," and Lemongrab puking in Peppermint Butler's face in "Too Young".
"Jake vs. Me-Mow" has two standout scenes:
Finn and Jake spitting food into each others' mouths after hearing about Wildberry Princess' assassin. Understandably, Finn is revolted.
The Gross-Up Close-Up of Jake's mouth while he scarfing down his pie.
Finn: Uhh, Princess, let's continue this briefing outside. I can't look at his mouth when he gets like this...
Barb the spider giving birth in "Web Weirdos".
Magic Man's house. The decaying rat in the corner, the mold on the ceiling, and the giant puddle of snot on the floor.
"Five More Short Graybles" has Finn and Jake sticking their thumbs into a Surinam toad that's giving birth. Eeuuugh.
"Mystery Dungeon" has Lemongrab contemplating eating the Ice King. The Ice King is an old, saggy man who probably smells bad. And Lemongrab wants to eat him. Eeewwwww. Later in the episode, Lemongrab eats a pie out of a giant rat's mouth.
Some of the Lemon Children in "All Your Fault" are rather grotesque. Special mention goes to the giant, fat, bush-looking thing that was drooling on the floor and saying "Blauh, blauh" repeatedly. Also the green, fat one who licked Jake's hand. Perhaps the grosses part of the episode was the scene in which newborn Seed-Wad vomits all over himself, and the vomit drips down his little chest and all over the table and floor.
The video Ice King sends Finn and Jake in "A Glitch is a Glitch" grossed out just about every viewer. What's worse is when Finn and Jake eat Finn's hair.
Morbidly obese Lemongrab in the episode "Too Old." Euuugh... No...
To say nothing of Lemongrab 2's state...
The intense drooling of bird Finn in "The Food Chain" and his death from malnourishment.
Among the most disturbing examples from the show that succeeds in making the characters themselves feel ill along with the audience is the virus-laden video in "A Glitch is a Glitch". For those who don't want to watch it here, it's a scratchy video featuring a pale, Samara-like girl with long black hair in a dimly-lit room covered in black mold. She then proceeds to stuff her hair into her mouth and slowly swallow it whole. Though that might not seem nasty, her pained facial expressions plus the metallic, guttural choking and retching noises she makes with each swallow makes it painful to watch.
Finn's foot rash in "Princess Potluck" is bad enough to make Jake want to puke the two times he sees it.
In "Breezy," the implied sex between Finn (an underage boy) and LSP (A lumpy mass with Pendleton Ward's voice).
In "Be Sweet," Marceline cleaning her navel during her phone conversation with LSP.
that's giving birth. Eeuuugh.