Casting Gag: The antagonists in the season 5 episode "New Bully on the Block" are Wolfgang (voiced by Toran Caudell, Arnold in season 1) and Ludwig (voiced by Phillip Van Dyke, Arnold in seasons 2 and 3).
The Danza: Arnold's mother, Stella, is voiced by Antoinette Stella.
Executive Meddling: In a rare positive example, the original ending to "Pigeon Man" was to have Vincent plunge himself off of the building, committing suicide, Nick execs didn't like this message of hopelessness and with a little tweaking we got Vincent flying off into the sunset, learning that there are some people who can be trusted.
Coach Wittenberg is voiced by James Belushi (better known as "Jim Belushi," brother of deceased Saturday Night Live cast member John Belushi. Jim Belushi actually was a cast member on SNL from the 1983-1984 season to the 1984-1985 seasonnote Jim Belushi lasted on SNL half as long as his brother, John, who was on the show from 1975 to 1979. He was the first SNL cast member to be fired, then rehired, and one of three cast members to have a brother who was also on SNL, joining Bill Murray [brother of Brian Doyle-Murray] and Dan Aykroyd [brother of Peter Aykroyd]. And, yes, that's the same guy from the sitcom, According to Jim).
One of Eugene's voices was both Numbah 1 and 2, as well as Sai. Eugene's original voice actor was Christopher Castile, who was on the ABC sitcom, Step By Step and was on an episode of Are You Afraid of the Dark? about a pair of stepbrothers who accidentally unleash the ghost of a convict while on a field trip to an abandoned penitentiary.
Arnold's Grandma Pookie is Charolette Pickles,Babs Bunny and Dot Warner (though the voice Tress MacNeille uses for the role sounds like Principal Skinner's mother [after the season one Simpsons episode where Bart goes to France and an Albanian boy comes to the Simpsons' house] and that crazy cat lady who first appeared on the season nine episode where Bart and Lisa are hired to anchor a kids' news show).
Christine Ebersole (of the 1981-1982 season of 'Saturday Night Live) voiced the short-lived boarder Lana Vail.
Eugene was played by four actors. Ben Diskin, his 3rd voice actor, lasted the longest at three seasons.
An interesting example with Curly is that they replaced, then reinstated his 2nd voice actor, Adam Wylie, twice.
Arnold himself has probably the most extreme example of this trope, with a grand total of six different voice actors! He stands out even when compared to the other characters, who had at most four (still a high number considering the show's lifespan).
One of the most egregious offenses was holding back "Parents Day" from a U.S. airing for 3 full years.
The often pre-empted season 5. According to That Other Wiki, Season 4 was 1999-2000, while Season 5 was 2001-2005. That's right: it took 3 years for one season! Craig Bartlett, the creator of the show, quit because Nickelodeon tried to force him to sign a non-complete agreement, which he rejected since he was also working on a show for Cartoon Network at the time.
Talking To Herself: Whenever Helga talked to Campfire Lass, or Sheena, or Stinky's girlfriend Gloria ("Helga's Boyfriend"), it was Francesca Smith doing both voices.
What Could Have Been: Helga was supposed to get her own spin-off set a few years into the future (around the time that Helga would be high school-aged), but the idea was axed because it would have been "too dark" for children's TV (in Nickelodeon's words, they said the spin-off would have been "...too much like Darianote though Daria never showed a miserable home life like the one the Patakis have. Yes, Helen and Jake sometimes put work before family and Jane and Trent are basically left to care for themselves because their mom is a hippie artist, they don't know who their biological father is, and the other siblings left for a better life, but it's not as painfully dysfunctional as Helga's family"). Considering the glimpses we got of Helga's miserable home life, rejecting the idea is either a wise decision to keep from traumatizing kids and incurring the wrath of Moral Guardians, or a foolish decision, as the Periphery Demographic of the show would miss out on an adult take on a kids' show that wasn't poorly-made, fan-written trash.
The "jungle movie", which would have seen Arnold find his missing parents.
It was already partially animated before being cancelled. A clip from the unfinished movie can be seen here.
In the first season, there was another tenant named Lana who was originally going to have a crush on Arnold, getting him to do favors for her and regularly making him uncomfortable by sexually harassing him. Nick execs of course found this too disturbing and axed the idea. Since Bartlett couldn't figure out a new direction for the character, Lana vanished after the first season (save for some later cameos in flashbacks or the animators mistakenly placing her into scenes at the boarding house).
Word of Gay: Mr. Simmons. His voice actor is openly gay.
Word of God: All that's canon is that Helga confessed to Arnold in The Movie, and there are plans to go the Jungle to find his parents.
Helga's mom is an alcoholic and those "smoothies" she drinks are just a child-friendly substitute for booze as Nickelodeon is, first and foremost, a kids' network and, despite having adult jokes in it for the Periphery Demographic, references to alcohol and alcoholism is best left to the adult shows on Nick at Nite (though there are mixed alcoholic drinks that exist as smoothie-type drinks, like the daiquiri, the hurricane, and the pina colada).
The scene where she asks Helga to pass her the Tabasco is especially telling, considering it's a Bloody Mary (or Maria, if Miriam prefers tequila to vodka) ingredient.
Arnold's last name? Turns out it's been said many times by Phil: Shortman.
The show takes place in the Pacific Northwest, most likely in Washington State.