Koosie making a pepé wrap is an allusion to Dom De Luise's second career as a cook.
Rasslor from the Dial M for Monkey episode of the same name was voiced by Randy Savage. His last line in the episode was, naturally, "OH YEAAAH!".
"Just an Old Fashioned Lab Song" is wall-to-wall references to guest star Paul Williams' music and personal life. note This includes: his height, his appearance, his songs ("We've Only Just Begun"), his album titles ("Here Comes Inspiration") his album covers (the title card is made to look like the cover of his album "Classics"), and even his car! Thankfully, they didn't include the substance abuse.
In the Justice Friends segment Things That Go Bonk in the Night, Major Glory and Puppet Pal Mitch, both voiced by Rob Paulsen, have puppets of each other, as do Valhallen and Puppet Pal Clem, both voiced by Tom Kenny.
Adored by the Network: Well not surprising considering it was one of Cartoon Network's first successful original animated series. But from 98-2001, it, along with The Powerpuff Girls were the flagship shows of the network.
Keep Circulating the Tapes: Bits of the first season were released on a now out-of-print VHS tape, the same goes for a collection of Genndy's favorite episodes. "Ego Trip" was released only on VHS, and that's out of print as well. The first season did get an official DVD release, but it does not include the banned "Dial M for Monkey" cartoon "Barbequor". Thankfully, the entire series (sans "Barbequor" and "Dexter's Rude Removal") is available on iTunes, and the entire first and second seasons (again barring "Barbequor") are available on Netflix.
"Barbequor" is available on Netflix in the United Kingdom.
As of May 3, 2015, all episodes (minus "Barbequor") are on Hulu.
"Rude Removal" became this after it was finally shown to the public. [adult swim] got its hands on the episode and actually aired it, and afterward posted the short on its YouTube channel in 2013. Shortly thereafter, the episode was made private and no official release of it currently exists.
Dexter's VA went from Christine Cavanaugh to Candi Milo when the former retired from voice acting early in the last two seasons.
Dee Dee's VA continuously alternated between Allison Moore and Kat Cressida.
Action Hank only gets to speak in three episodes, but his voice actor is never the same. He is played by Michael Armstrong in "Beard to be Feared", John Garry in "Decode of Honor", and Kevin Michael Richardson in "911".
In the "Dial M for Monkey" shorts, the chief of the organization Agent Honeydew works for was originally played by Robert Ridgely, but Earl Boen became his voice actor in the second season.
Monkey's sound effects were done by Frank Welker, but his speaking voice in "The Lab of Tomorrow" was provided by Corey Burton.
Hungary holds a reunion for the Sailor Moon cast: Dexter's first voice was Melvin; Dee Dee is Chibiusa; Dexter's first mom was Sailor Neptune; Mandark's first voice was Alan's second voice; and Dexter's second mom was Berthier, Karonite, Eugeal, Mimet, Telulu, Palla Palla, and Fisheye. Mandark's second voice is not Chad.
Frank Welker does this whenever both Monkey and Quackor face off.
Eddie Deezen voiced Mandark and his sister Lalavava.
Throw It In: The reason for the audience randomly flying away at the end of the Big-Lipped Alligator Moment song in the Chubby Cheese's episode is because the storyboard script had a note saying "audience takes off". The Korean animation studio took this literally and had the audience "take off" into the air. The editors shrugged and added in a magical sound effect to accompany it.
Despite the science theme of the series, you rarely if ever see mentions to the internet. While the internet did exist in the late 1990s, it didn't become extremely mainstream until a few years later.
What Could Have Been: Eddie Deezen's laugh for Mandark began as a flat, smug "Ha," lifted from one of his standup bits. When he was told that they wanted a "mad scientist" laugh, he gave them the now-famous "HA ha ha! HA ha HA ha HA!"
Word of Dante: Some people who got to see "Rude Removal" at conventions claimed it contained more swearing than it actually did and it was unbleeped, but Genndy insists that the version released by Adult Swim is the only version and those people are either misremembering or intentionally making stuff up.
Word of God: Genndy has stated that the infamous episode "Rude Removal", where Dee Dee and Dexter go into a machine that removes all of their rudeness and concentrates it into another version of themselves, was actually meant to be part of the the show's second season, not just a way for the crew to blow off steam during a stressful production. The network, however, disapproved of having an episode that centered around the characters constantly swearing, censored or not.
Writing by the Seat of Your Pants: Genndy Tartakovsky, who was at the peak of his workaholism at the time, claims that the original show was produced so quickly that he didn't even see the finished product until it was on television.
Write Who You Know: Dee Dee and Dexter's relationship was based on creator Genndy Tartakovsky and his brother Alex, who was always destroying Genndy's personal projects when they were younger. Ironically, said brother eventually grew up to be a scientist!