These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crosses the Line Twice: The Christmas special. Features such things as a mall being burned to the ground as the Mall Santa is mercilessly beaten (with The Grim Reaper implying that this happens all the time and Billy blaming it on the sad state of the economy), evil vampire Santa, Santa's evil vampire wife, and the school bully being eaten by a pack of flying vampire elves. An average Christmas Episode of a show is packed with heartwarming moments and the old theme about "family and friends being the true meaning of the holiday season," but this show proves that you can have none of that and still be entertaining. Combine that with the fact that this is a children's show (by Cartoon Network's more-or-less loose standards) and you get a truly dark (and hilarious) episode.
A lot of the humor can dip into this, it being Black Comedy.
Lord Pain towards Grim in "House of Pain". He kisses his feet, pleads to be his slave and strips in front of him to show his Grim tatoos.
Irwin and Nergal Jr. were both rejected by Mandy. At the Prom, they decide to dance together without hesitation.
Billy to Grim: "Now bend over and let me RIDE YOUR HUMPS!" He's also admitted love to Grim.
The ending to "Billy and the Bully" is rather Ho Yay-ish, having Sperg and Billy do such things as hugging tightly, riding a tandem bike, and skipping happily through a flowery meadow together.
"Company Halt": After Hector Con Carne finishes explaining his giant rubber band attack plan, General Skarr exclaims, "I love you SOOO MUCH, Hector!" and hugs him (or rather Boscoe, to whom his brain is affixed) tightly. Hector responds with, "Yeah, I...love you too, Skarr." as Boscoe hugs Skarr back and makes affectionate noises. Ghastly backs away slowly in awkwardness.
Dean Toadblatt makes out openly with Squidhat and the two are wed in the same episode.
Billy, though a good kid, has tried to kill his "adopted" son just because he's afraid of spiders.
Irwin has allowed a Pyramid to be built in his honor, regardless of whether or not everybody remains brainwashed.
Nergal forces a number of people into transforming into his "Friends" in the second episode he appears in.
Boogie crossed it in Wrath of the Spider Queen. During the contest for who would become the future Grim Reaper, Velma was close to winning, but Boogie decided to rig the voting. When Velma finds out, she thinks Grim tried to cheat, costing him his only friend.
Or when the closing iris closes on Billy's head and a bunch of blood and snot sprays from his nose and his eye pops out.
Or when Billy and Mandy force Grim to resurrect the school mascot cats. That, in itself, is pretty bad, but when Billy asks one of the cats to scratch his back...his skin and muscle get shredded by the splintered claws of the undead cat, and his spinal column pokes out.
In the episode, "Zip Your Fly," Mandy accidentally swapped her head with that of a fly after messing with Grim's zipper. That's pretty standard for the show, but then after losing the zipper to the dump, she ends up eating the entire landfill before she can find it and get her body back.
The Scrappy: Billy is this to more than a few viewers, due to how arguably flat he is as a character and for being a often pointless Toilet Humor dispenser. He's even this in universe.
Fred Fredburger is also this in universe, even managing to annoyBillywith his stupidity. Among viewers, though, he's a Base Breaker, with many who love him and many who feel he ended up over-used. (Although he wasn't half as over-used as characters like Cheese and Goo.)
Or Junior and Mandy, to a lesser extent than the other two.
Unfortunate Implications: Billy, according to the Christmas Episode, is Jewish. He's the character with the large, bulbous nose. This may be why it's never brought up again. note Though the standard Jewish stereotype is a large, hooked nose. Close enough.
Sperg is a bully with implied mental health issues. The word "sperg" is also used as an ableist slur towards people with Asperger's Syndrome, who are commonly accused of faking their disorder as an excuse to bully people.
What Do You Mean, It's for Kids?: Like nearly all of Cartoon Network's in-house programming (specifically those from the "Cartoon-Cartoon" era of shows), this show is a kids' cartoon on the outside, and on the inside, is packed with a lot of things that most normal channels wouldn't allow to touch children's animated shows. In the case of this show, it's Black Comedy, some mild sex and toilet humor, and disturbing imagery that makes Invader Zim look like My Little Pony.