The exchange in "Cement Heads" when the gangsters are attempting to trade Ickis for the stolen toenails.
Gromble: No deals with hostage-takers, Chimera. Ickis: Whaddaya mean no deals with hostage takers?! What's this world coming to?! If you can't deal with a hostage-taker who can you deal with?!
Somewhat surprisingly, this actually causes the Gromble to relent.
Oblina’s runaway brain in “Who’ll Stop the Brain?”
"It's Just a Movie" in its entirety. Basically, Ickis goes to a theater and watches a cutesy kid's film about a fluffy Teddy bear named Cuddles (an obvious parody of the Care Bears). Being a monster, and thus being afraid of cute, cuddly things, it ends up scaring him, and throughout the episode, Ickis is freaking out about Cuddles the Bear finding him. And then the real Cuddles (actually just Zimbo in a suit) appears and begins to chase Ickis around, threatening to cuddle-wuddle with him.
What's funnier is that Cuddles is voiced by Tim Curry (who already does Zimbo) of all people.
The scene where Ickis tries to frighten a nurse but she instantly forgets her fear and starts baby-talking and cuddling him upon seeing him seems to be one In-Universe; even the Gromble finds it so hilarious he can't stop rolling around on the floor laughing, despite how much a failed scare angers him.
The episode where all the monsters become obsessed with a lava lamp. Or as they call it, "The Laaaahmp!... @_@"
Gets a Callback later on in "Battle of the Century", where Simon uses it to distract the trio and Gromble.
The episode "Monsters Are Real" has Ickis photographed and needing the other monsters who help him by confiscating all newspapers with his photograph printed in them. Two monsters collect newspapers in one of those newspaper dispensers where you pay a nickel to get a paper. One of the monsters insists on paying for every individual newspaper. When told to just take all of the papers out at once, the monster replies that it wouldn't be the right thing to do.
The episode "Room Without a Viewfinder" has Ickis attempt to remove the Viewfinder so that the Gromble won't know that he has been slacking. The Viewfinder ends up coming to life and Hilarity Ensues. After recording the memories of a corrupt mayor, a cheating boyfriend, a Small Name, Big Ego chair salesman, and an old man, the monsters eventually return the Viewfinder. The episode reaches its funniest in the ending, where Ickis is put in the Viewfinder and the humans' memories are mixed up, resulting in such hilarious images as the mayor being made fun of by children for losing a game of musical chairs and the chair salesman's younger self betting on a horse race.
Gromble: Ickis, why do I get the feeling that this has something to do with you?
A Parental Bonus in "Krumm Gets the Dreaded Nolox" when Krumm hiccups up a See 'n' Say-like toy:
From the very first episode, when the trio venture out onto the surface on Halloween to go trick-or-treating. They go from house to house gleefully yelling "Prick your feet!" because they mishear "Trick or treat!", and they fail to understand that you're not supposed to ring the doorbell for more candy after someone closes the door.
"Look, kids, it was cute the first time — but now you're just pressing your luck!"
One episode shows a brief overview of the history of monsters, beginning with one of the cave-monsters, who, lacking a large number of available humans to scare, had to keep scaring the same caveman repeatedly.
From the same episode, Ickis learns his dad broke the Liberty Bell, and Krumm despondently says his dad never broke anything important. The Gromble responds "Yes he did. My will to live."
Most of the episode "Fistful of Toenails" from Ickis and Oblina's fake accents (while Krumm still speaks in his regular voice) to the stinger at the end with the Gromble.
Luggage Attendant: Where are you headed? Tourist (voiced by Bill Fagerbakke): Milwaukee, Wisconsin! Luggage Attendant:(impressed) Oh boy, sir, you are in for the thrill of a lifetime!
Millionaire: What would you say if I offered you $1 million to have your wife... make me a cheese omelette? With fine Wisconsin cheddar, of course.
The reveal that the mincing fashion designer with the over-the-top French accent turns out to be some guy named Leonard from Duluth.
"Monsters Don't Dance":
Manager:(to worker stacking toy boxes) No no no, not the lizards! How many times do I have to tell you: Lizard out, Murray in? I've got Murray coming, and this kid puts out lizards. Why did I hire you?! Worker: Sorry, dad.
Later:
Oblina: This imposter [Murray] is spreading love and happiness everywhere, and he must be stopped!
,” which sounds like Charlie Adler imitating Gilbert Gottfried, and even got released as a track on the “Best of Nicktoons” compilation CD