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.
Also Arnold himself at the end of the episode "24 Hours To Live".
Curly, with the emphasis on crazy (and not a "cool, kooky" crazy either. More like a "This kid needs serious mental help" crazy). To elaborate, he once broke some animals out of a zoo - alone. Just because he wanted to.
I'm going with the idea that Arnold considers the mooning, while still offensive, is less so than betraying his friends; he might have said something to them later about not doing it again. He does have a strong sense of honor and probably would have spoken up had they been stealing or vandalizing school property. But it's still a bit disappointing.
Fan Fic Fuel: We have enough details about the Jungle Movie and The Patakis that many fanfic writers have written their own versions.
"Funny Aneurysm" Moment: At the end of "The Flood", as floodwaters surround PS 118, Principal Wartz stands on the roof of the school singing, "Way down yonder, in New Orleans..."
Harsher in Hindsight: It was once implied that Helga's dysfunctional home life was the end result of her sister Olga moving out (as her parents favored Olga over Helga), leading her dad to push Helga to be more like her older sister while Miriam became a covert alcoholic (or "smoothie" drinker) just to cope with dealing with a Jerk Ass like Bob and a bratty daughter like Helga. The episode "Helga on the Couch" revealed that Helga's been neglected by her parents since she was preschool-aged and that her secret crush on Arnold was the first time anyone has given her any sort of affection in her life.
Jolly Olly Man, the ice cream vendor though this troper had a hard time understanding his frustration caused by only a boring job, and his problems keeping it which may have something to do with his jerkassery which delves into Nightmare Fuel. It's not good for the business if you act like a raging psychopath even with your customers. And he had no idea what's wrong with his behavior. Arnold had to teach him the basics of common courtesy.
Memetic Mutation: "STOOP KID'S AFRAID TO LEAVE THE STOOP! STOOP KID'S AFRAID TO LEAVE THE STOOP!"
Never Live It Down: Iggy. In most episodes, he's simply a background character, with only one or two notable lines. "Arnold Betrays Iggy", his A Day in the Limelight, will forever have him be known for all of the (well-deserved) negative tropes on this page.
An even bigger Scrappy than Lila who only had one major appearance is Iggy from the episode "Arnold Betrays Iggy". It doesn't help that the episode itself ends on the note that Iggy has gone beyond Arnold's forgiveness.
Gerald's little sister Timberly gets a lot of hate in fandom mainly for being really annoying and is usually hindering Arnold and Gerald's plans in any episode where she plays a key role. The best example of this is the "Chocolate Turtles" episode.
Seasonal Rot: Seasons four and five aren't considered fan favorites, mainly due to Arnold's character becoming bland and the Arnold/Helga/Lila love triangle becoming a Romantic Plot Tumor (contrast to Season 3, where it only came up twice and actually seemed to be resolved within those two episodes!) There were still some good episodes here and there, though, particularly "Parent's Day" and "The Journal."
Unfortunate Implications: The frequent aesop that one is heartless for not returning someone's romantic interest. Be it Arnold towards Lila, Rhonda towards Curly, or Lola towards Ernie. More or less averted with Helga and Brainy and Lila and Arnold, though.
What an Idiot: Helga probably could get Arnold's interest if she was nice, but she rarely gives it a try.
Of course not. She has a rep to maintain. Harold even made fun of her on day one (when Arnold first noticed her) in a flashback in the episode, "Helga on the Couch".
Arnold realizing that Iggy won't forgive him and instead of letting go, he suffered through a Humiliation Conga.