In the episode "Turtle Temper", Raphael (voiced by Sean Astin) is forced to go home by the other turtles because his temper got the best of him, but came back when his brothers are attacked by Spider Bytez, who was actually the mutated Vic. Does this look familiar?
All-Star Cast: Aside from some legends in the voice acting community (Rob Paulsen in particular makes his return to the Ninja Turtles after twenty years), the show also has legitimate movie stars (Jason Biggs, Sean Astin, Kelly Hu, Seth Green) slumming it to play cartoon turtles. Mae Whitman as April, Greg Cipes as Mikey, and round out the cast with lower key TV and movie Stars. Then again when Andrea Romano is working with the voices, this should be no surprise.
Approval of God: Stan Sakai approved of Yuki Matsuzaki's performance as Miyamoto Usagi.
Ascended Fanon: Both Leo X Karai and April X Donnie were popular but completely crack pairings. This show is the first to put explicit Ship Tease between them.
Corey Feldman (Donatello's VA from the first & third live action films) voices Slash, the mutated form of Raphael's pet turtle Spike. This is actually a twofer, as aside from Feldman's previous work in the franchise, Raphael is voiced by Feldman's former Goonies co-star Sean Astin.
Kevin Michael Richardson previously voiced General Aguilla, the Big Bad of the 2007 TMNT film.
Renae Jacobs, who voiced April in the '87 cartoon, is voicing April's mother, or rather a monster disguised as her mother.
Minae Noji, who played Karai in the the 2014 film voices Miwa/Karai's mother, Tang Shen.
Dracula in the Halloween arc is voiced by Chris Sarandon, who played the vampire Jerry Dandridge in Fright Night (1985).
Channel Hop: The "Mutant Apocalypse" and "Monsters vs. Mutants" tales of season 5 were televised exclusively on Nicktoons, before the show's finale, "Wanted: Bebop and Rocksteady", premiered simultaneously on both Nickelodeon and Nicktoons.
Creator's Pest: Ultimately subverted. The creators mentioned that they began to hate Dogpound, to the point they nearly killed him off in the second season. They ultimately decided to turn him into a new incarnation of the character Rahzar instead.
Creator-Preferred Adaptation: According to executive producer Brandon Auman, franchise co-creator Kevin Eastman considers this series to be the best version of the Ninja Turtles.
Dawson Casting: The youngest Turtle voice actors are Greg Cipes (31) as Michelangelo, followed by Jason Biggs (35) as Leonardo, Sean Astin (42) as Raphael, but the winner is Rob Paulsen voicing Donatello at age 57. Leo's follow-ups Dominic Catrambone and Seth Green (40) also count. Mae Whitman comes closest to her character April's age at 25. note If she was playing a traditional adult-aged April she'd be just right.
Executive Meddling: Ciro Nieli says that he really wanted to use The Flaming Carrot, but Nickelodeon told him that "nobody wants to see a guy with a carrot for a head".
Nickelodeon aired "Raphael: Mutant Apocalypse" in the middle of Season 5, even though it was originally intended to be the series finale, before declaring it an alternate dimension. The 1987 crossover arc aired as the final episodes on Nickelodeon instead.
Fake Brit: When Doctor Rockwell gains his intelligence back and can speak, we have American actor Tom Kenny giving him a British accent.
People have guessed that Spike, Raphael's pet turtle, due to his name and being a Turtle, would wind up being mutated himself. This thread, featuring concept art from Nick Magazine, confirms that Spike will become Slash.
The revelation that Karai is in fact Miwa, Splinter's biological daughter. The fact that character bios (likely when the series was still in development) were leaked and indicated Shredder had a daughter named "Miwa", and that Splinter seemingly also had a daughter, without naming her, likely helped this process.
Before Slash's debut his voice actor claimed that Slash would be an ally to the Turtles, and the perfect companion to Raph, a description that would come to fit better Casey Jones. And then "Newtralized" happened where Slash did make a Heel–Face Turn and become an ally.
Some months before "The Wrath Of Tiger Claw" aired, some scenes from a storyboard were leaked on tumblr where Splinter tells Karai that she is his daughter and later it is seen her confronting Shredder about this. When people asked Ciro Nieli if these scenes were real, he said no. Comes the episode with this same exact scene and now people is left wondering, specially because the other leaked scene was a kiss between April and Donatello that also turned out to be a real scene come the Season 3 episode A Foot Too Big.
At Comic-Con 2013, Ciro Nieli claimed Irma would be Mikey's love interest. Unless there's a real Irma out there, there's no chance of that.
Orphaned Reference: At one point "Stockman Industries" shows up as a Freeze-Frame Bonus in Season 2, but so far no mention has been made of Stockman actually owning his own company.
Dominic Catrambone voices Leonardo in all Season 2 episodes produced after "The Wrath of Tiger Claw" instead of Jason Biggs (due to his wife's pregnancy), just as he voiced him in the video game. After that, from Season 3 onwards, Seth Green takes over the role, where it is explained in-universe that Leo received throat damage in his fight with Shredder (in addition to Biggs's wife's pregnancy, Biggs also made jokes about the two missing Malaysia Airlines flights in 2014 plus sex abuse jokes about The Pope and mocking the death of a Bachelorette contestant, comments that likely cost him the Leonardo role and led to Leo superfan Green replacing him).
Professor Rockwell's actor went from Frank Welker to Tom Kenny in Season 3, though this is more because Rockwell has finally regained his intellect and can talk again.
Kraang Prime is played by Roseanne Barr in Seasons 1 and 2, but by Rachel Butera in Season 3.
Joan Grody has gone through three different voice actors in the Hebrew dub.
Even considering James Avery's passing in 2013, the 1987 Shredder is played by Kevin Michael Richardson in season 5's three-part crossover, making him the seventh actor after Dorian Harewood, Jim Cummings, Townsend Coleman, William E. Martin, and Load Williams, to have portrayed that incarnation of the character.
This series is pretty bad about this. Release dates of the episodes have a somewhat bad tendency to jump around to another date or be several weeks or even months after the most recent episode, and the show has a really bad tendency to almost always go on at least a month-long hiatus after airing three to four episodes. Particularly noticeable with "Parasitica", which had been announced to be a week after "Pulverizer Returns!", but then got delayed to June 1st, and finally aired on July 20th. The episode was, however, aired in other countries, but went through similar problems.
Happened again with "Turtles in Time" which, after a month long hiatus, was announced to air on June 28th, 2015. When the episode airdate came, "The Deadly Venom" was rerun instead, and the wiki relisted "Turtles in Time"'s airdate back to "To Be Announced." It finally aired on August 2nd, 2015.
The 4th season had a serious problem with Schedule Slip, with the "City at War" arc starting about 6 months after "Earth's Last Stand", and having a major break during the fall. After three wham episodes, another lengthy hiatus began. Season 4 has had the longest airtime of all the seasons thus far, having started in fall 2015 and ended in February 2017, and was upped from a 20-episode slate to the previous seasons' 26. However, the final four episodes of the season had aired in South Korea in December, weeks before they were aired in the United States in February 2017. This case of Schedule Slip mixed with those airings led to spoilers regarding the end of Season 4 surfacing on a few sites such as Tumblr, which infuriated showrunner Brandon Auman, who wrote those episodes.
Self-Adaptation: Stan Sakai, the creator of Usagi Yojimbo, wrote the three-parter in Season 5 that dealt with that series' world and characters.
They made the four turtles non-identical for the first time to reflect the show appearance. Also worth noting is a line of zipline playsets coinciding with season 2, which used more ziplines and even put them in the OP.
Also in the OP and several eps is Mikey's skateboard, which has the same colors as the Sewer Spinnin' Skateboard toy... at 1/8scale.
For some strange reason Kirby's figure of mutant form is named "Kirby Bat" even though Mikey named it "Wing Nut" in the series itself, much to Donnie's displeasure. What makes it so odd is that this means they chose to use a Late-Arrival Spoiler over keeping the preexisting characters name, see the Mythology Gag spoilers. However, Mikey does use the "Kirby Bat" name in "The Lonely Mutation of Baxter Stockman".
There are a few scenes that indicate that the writers have done some research, like the difference between "Venom" and "Poison", and how a Wasp dies when the stinger breaks off, though Artistic License does still factor in (like the latter being a Parasitic Wasp, which are one of the types of Wasps that don't die from their stingers breaking off).
Also, Japanese culture references are, for the most part, surprisingly accurate and tasteful, and are very rarely used exclusively for Rule of Cool. Even if subtle moments, like when Splinter is shown meditating in "Of Rats And Men"; he uses real Shintoist hand gestures (or "mudras").
Talking to Himself: Done in "Trans-Dimensional Turtles" as a gag with Rob Paulsen. The 1987 Raphael and the 2012 Donatello exchange some dialogue in which they insist it's the other one that sounds weird.
Torch the Franchise and Run: Ciro Nieli admits the "Mutant Apocalypse" three-parter, which features the Turtles failing to stop a Mutagen bomb in a flashback and the heavily implied deaths of almost all of the supporting cast, was done so that nobody could continue this series after it was finished. On the other hand, other crewmembers and Nickelodeon themselves seem to disagree with that assessment, with the last outright calling the story an alternate universe, rendering the arc non-canon.
Typecasting: Rob Paulsenhimself has gone on record saying that if you have to do a TMNT revival, then you also have to have Greg Cipes as Michelangelo. Even before that, Michelangelo holds the same role in the Turtles that Beast Boy (Also played by Cipes) held in the Teen Titans.
Corey Feldman (Donatello's VA in the first & third movies) auditioned for the role of Raphael, which would've added a whole new level of meta-humor to the casting by having Feldman & Paulsen switching Turtles from the last time they worked in the franchise.
Character bios were leaked when the show was still being worked on, and Shredder's stated that he had a daughter named "Oroku Miwa" who allowed him to stay human. The name "Miwa" is used for the name of Splinter's daughter in the show (Splinter's bio still mentions a lost daughter, though suspiciously leaves her unnamed). The final show instead uses Karai as Shredder's adopted daughter in keeping with last televised incarnation, albeit it turned out that Karai was Miwa the whole time.
Some of the first concept art reveals that it was planned for Raphael to be the one to have the crush on April, but later the screenwriters decided that it suited more Donatello.
One piece of rejected concept art showed that there were once plans to have Wingnut and Screwloose appear on the show as Expies of Batman and Robin, but instead Wingnut became nothing more than a nickname for Kirby O'Neil's mutant bat form and Screwloose was never given a mention at all. However, come Season 4 the Batman & Robin-esque versions of Wingnut and Screwloose do make an appearance.
The production team actually began to dislike Dogpound so much they seriously considered killing him off in season two, but they declined, instead mutating him into this series' incarnation of Rahzar, who they had wanted in from the beginning.
An Asian actor was considered for the Shredder, but the crew couldn't find an appropriate actor for the character, so Kevin Michael Richardson was cast.
When asked on the possibility of more seasons, it was noted that while Ciro Nieli planned on finishing the series with five, they weren't opposed to doing more, even if it'd mean they would have to modify the series plan. No such modification occurred, and the series ended at five.
Originally, Super Shredder's decapitation in the season four finale would have been more obvious, with a very thin gap between the head and torso in the black-and-white shot. According to Ciro Nieli on Instagram, Nickelodeon caught the gory detail and censored the gap in all showings of the episode.