- I Knew It!: That Alternative is a prequel to Classic and Progressive.
- The Other Darrin: Interestingly, Megumi Hayashibara voices Haruko (or rather, "Haruha") in Progressive, despite Mayumi Shintani reprising her role as Haruko in Alternative and Kari Wahlgren reprising the role in English for both seasons.
- Production Posse: Production I.G and Toonami previously collaborated to produce an original anime for Toonami, that anime being IGPX: Immortal Grand Prix.
- Role Reprisal:
- Mayumi Shintani and Kari Wahlgren return as Haruko in their respective languages, though in the former's case, only for Alternative.
- The cat cameo from the previous series even had the original voice actor back...for one meow line.
- Sequel First:
- The first episode of Alternative aired subtitled in the U.S. as an April Fools' joke, two months before the first episode of Progressive.
- In Japan, Alternative aired on September 7, 2018, while Progressive aired on September 28, 2018. Though from the interviews, Alternative was actually meant to come first. This is the reason why the Alternative cast shows up in Progressive's ED along with that season cast and original.
- Sequel Gap: Progressive and Alternative were made 18 years after the original FLCL first aired in Japan, and 15 years after its U.S. airing.
- Short Run in Peru:
- Progressive premiered in the U.S. on June 2, 2018, over three months before its Japanese airing.
- The first episode of Alternative takes this Up to Eleven with the subtitled version having a stealth premiere in the U.S. on April 1, 2018, five months before its Japanese airing.
- Star-Making Role: Jinyu became this for Allegra Clark, very similar to how Kari Wahlgren gained stardom after voicing Haruko in the original FLCL.
- Streisand Effect: On October 1, 2018, Digibro uploaded a video featuring a criticism of Progressive, which was later taken down by Production I.G themselves. Needless to say, this riled up both his fans and critics, who believe that Production I.G Can't Take Criticism. They eventually relented, and has since put the video back up.
- Word of God:
- According to executive producer Mitsuhisa Ishikawa, some students from the original series would return and be married in Progressive. We never do see any of these married characters after the thirty years (or so) Time Skip beyond seeing cameos of Naota and Mamimi in the credits...and knowing that Commander Amarao married and had his son Marurao.
- With the Gainax Ending of Alternative and tweets from staff, it's confirmed that the series in particular was intended to be a Stealth Prequel, and created a set up for an unused concept from the original FLCL where the small town of Mabase was actually on a colonized Mars the entire time.
Trivia / FLCL Progressive & Alternative