The citizens of Lundgren became this a little while after "Huggle Day"; some didn't have any problem with them turning against Sanjay and Craig (it is a part of the plot) while others felt they where way too OOC and feared that they're going to stay like that throughout the rest of the series.
Ronnie Slithers himself is considered this too, due to his actions in his debut episode. with that said, there are also fans who forgive him for said-actions and find him an interesting new character to shake things up.
Leslie; he's an annoying and bizarre antagonist with a sympathetic backstory and a few funny moments, naturally leading to folks not really knowing what to make of this guy.
Sanjay himself heavily suffers from this. Is he a nice kid who is also a great leader or an annoying, unappealing, poorly-adapted character who never learns anything. The latter is mostly viewed by haters of the series and it didn't help that in early episodes, he was more childish and clumsy...then again, it might boil down to how you feel about this show given he's one of the two protagonists.
Even Craig himself isn't safe from this. You'll either like him for being a very unique character with a solid sense of humor, or you'll dislike him for being an annoying neglectful Jerkass who treats others badly, including Sanjay. Some episodes like Heartyface tend to give him a more mean-spirited/antagonistic part of the character, though others show that he's actually a nice guy who really cares about Sanjay. Doesn't help that he's voiced by Chris Hardwick.
Bile Fascination: Its status as one of the more obscure (and until Season 2, one of the more grossout-heavy) Nicktoons draws curious newcomers in to see how bad it really is; a good number of them turned out to be satisfied with how the show became.
Broken Base: The entire series itself, probably one of the biggest ones in the Nicktoons Canon. Either it's one of Nick's most unique and underrated classics, with a creative suburban world, interesting and diverse characters, and for giving out some solid storylines and character development (even if only less than 20 characters get a decent amount of spotlight), or it's one of the most generic, clichéd shows that suffers from wasted potential, unnecessary gross-out humor, poor characterizations, mean-spirited writing, and constant animation errors.
Estrogen Brigade: For starring in a show whose primarily cast in boys and targets such with its Toilet Humor and "bro code" dynamics, Sanjay and Craig themselves have quite a number of fangirls, to the point where there's even self-shipping fics/art from the target audience back when the show was still running.
Megan is actually adopted, since her father was never seen or mentioned in any episode. When she was first born, Megan was lost by her original parents and was later found by Debbie Jo. Since then, Megan was raised to be a triple-threat pageant winner.
In various fanworks, Sam Lastnamè is written as a more major character where she is one of Sanjay's closest friends. (In the actual show, Sam only appears in 7 episodes throughout Seasons 2 and 3. She only makes a major role in two episodes, the other five are non-speaking background roles.)
10 years before the show premiered, the first issue of Yotsuba&! was published and released; the latter's central character is Yotsuba herself: a green-haired girl who sometimes wears a red-sleeved baseball shirt, making her look like a genderbent gijinka of Craig.
There's an OC based off of the titular duo named Lalasa, a Robot Girl (if not just a cyborg) who was introduced on September 2015 via the Beochan AU blog on Tumblr; fast forward a few months later and the show itself would have its own canon Robot Girl character by the name of Bertha Van Weld in "Partybot Returns".
Craig and Sanjay tend to be rather...close at times, special mention goes to "Kerplunk'd", "Doom Baby", and "Fart Baby" where the former is typecasted as the father in contrast to Sanjay playing the more delicate and motherly role; doubles as No Yay if you're sensitive about Interspecies Romance.
Penny's drama with Chicken Chuck throughout "Hot Sauce Boss".
Jerkass Woobie: Mr. Noodman will do anything to kill Craig and prevent Sanjay from having fun. But when you see how his father treats him during "Family Re-Noodman", you can't help but feel sorry for him.
"Barfy's Babies" tried to have a Tear Jerker moment with one of Barfy's children in danger of not getting adopted but then it started getting gross and then just plain weird when that puppy turns out to have the talent of becoming a Humongous Mecha that wouldn't look out of place in a anime involving such things.
The ending sequence to "Fart Baby" definitely counts, trying to mix in a heartwarming vibe and grossout as Sanjay and Craig watch the title "character" grow up and blow up in Leslie's face.
Nausea Fuel: For starters, Sanjay and Craig get covered in bird poop in "Laugh Quake".
The partner episode to that revolved around a giant ass transplant. It's about as pleasant as it sounds.
Even better is Sanjay's neighbor's attempt at a Sexy Walk with his bubble butt.
Pretty much the entirety of "Unbarfable".
The fact that Mr. Noodman not only unwittingly ate snake eggs, then vomiting out a lot of baby snakes from his mouth and nose, in "Family Re-Noodman".
Not to mention that quick scene where Sanjay and Craig where twerking together in "Susan Loogie".
The first season, which placed an more of an emphasis on the gross-out humor compared to the later seasons, really defined the show in many people's eyes, particularly certain episodes like the infamous "Fart Baby" (which even fans of the show tend to agree was an awful episode), to the frustration of fans who hold true that the show got far better in its last two seasons and wish others would give the show a second chance.
Ronnie Slithers' actions in "A Tale of Two Slithers".
It's very likely that the empathetic fans are forever haunted by Sanjay and Craig getting kicked out of town in "Huggle Day".
Ron the Death Eater: Sanjay and Craig themselves have been seen by haters as idiotic, hurtful, sick sociopaths, when in reality, they're both actually nice, smart, well-mannered boys, even if they can be pretty rebellious at times.
Some Anvils Need to Be Dropped: "Road Pizza" tells us to never force your opinions onto someone else. Given how politically polarized that modern times can be...
So Okay, It's Average: The series as a whole. The wider consensus of it, which agrees that while it's far from the worst Nicktoon and actually has a few good points, its gross-out humor, animation errors, and the wasted potential of the supporting characters are thoroughly underwhelming. The show's first season tends to be regarded as this nowadays (outside of widely hated episodes like "Fart Baby").
Sam Lastnamè. She is so beloved by the show's tiny fandom, that she is prominently featured in a lot of fanworks as one of Sanjay's group of friends, as well as being Megan's best friend. Sadly, in the actual series, she only appears in about 7 episodes throughout Seasons 2 and 3. Only two episodes (Butts Up and G.U.T.S. Busters) have her in a major role, the rest of them only have her in background roles. It's gotten to the point where it is believed that she is the mascot of this trope.
Herman Mandochini from Season 2's "Space Race". He was formerly Vijay's best friend in collage who had some affection for Darlene. But when Vijay was given Darlene's number, Herman became very enraged and turned against his friend. Afterwards, feeling betrayed by Vijay, he finished a rocket project (which he worked on with Vijay) on his own, and ended up in space for years. He never appeared on the show again after that episode, besides a cameo in the end of "Booyah for Bollywood", where he is back on earth for unexplained reasons.
Noodman's Girlfriend, Bernice, from the very debatable Season 2 episode "Fartwerk", for the same reasons as Herman. She too also made an unexplained cameo in the series finale.
Sanjay's parents, being part of the main cast, are well developed and show up in most of the episodes of the series. The other kids' parents, on the other hand, only show up scarcely in some episodes of each season, usually in non-speaking cameos and background roles. (Belle's father, Penny, on the other hand, is quite relevant on the show, due to the fact he is the owner of The Frycade, which made the character easier to write with.) Sam's Dad only appears in G.U.T.S. Busters for only 5 seconds with only one line, and never showed up again.
Tyson, while one of the most recognizable supporting characters on the series, is very underdeveloped and appears quite infrequently. It only gets worse in the final season where he only had one speaking role and the rest of his appearances are just cameos. Quite a shame, since the character proved that Mr. Noodman wasn't the only foil the the main duo.
Uncanny Valley: Carrying off of Jay Howell's previous work on Bob's Burgers, the chinless, muppet-like character designs can take a while to get used to.
Unpopular Popular Character: The titular duo are both considered trouble making, untalented weirdos by quite a few characters (including their own friends and family)...now, go ask the fans of what they think of Sanjay and Craig; to sum it up, they're pretty much Jenny all over again ten years later.
Unintentionally Sympathetic: Sanjay and Craig, in "Huggle Day"; they almost got the holiday canceled and the townspeople did have a reason to be angry at them...but a lot of watchers felt absolutely horrible for them.
Vindicated by History: It hasn't been fully hit with this trope yet, but when the show first premiered, everyone quickly wrote the show off as just another crappy post-Avatar Nicktoon with no substance (and to their credit it did kind of start off on the wrong foot). As the show went on, though, it began to show that it was more than thirty minutes of gross-out antics, fleshing out several of its characters, and telling some surprisingly well thought out and funny stories. By midway through the second season, the show had amassed a noticeable fanbase and was regarded as diamond in the rough in post-2009 Nick. Cementing its status as this was its untimely cancellation, which was met with disappointment from its fanbase (as can be seen here on this Tumblr post). At the time of the show's premiere, you would have been the laughingstock of most internet communities if you suggested that Sanjay and Craig would be missed at the time of its cancellation.
Sam, the little girl who was gonna lose her bird to Leslie in "Butt's Up".
Megan is considered one, especially in episodes like "Release the Craigan", "Susan Loogie", "Glory Hounds", and "Romper Chomper".
Sanjay and Craig themselves have their moments too, especially in "A Tale of Two Slithers", "Glad to be Sad" and ESPECIALLY "Huggle Day".
From the aforementioned episode, Huggle Bunny and Dolph Lundgren; it's revealed that the former's carnival performances where so he could give everyone gifts on his holiday but since everyone in Lundgren only thought about getting his gifts from him rather than giving to each other, Sanjay and Craig's fluke driven him to run away and take back everyone's gifts while the latter believes that he failed to bay his friend back for the shelter and simple gifts the former had given him after finding him in the snow for lord-knows-how-long.