Gedächtnis: The original? Never read it. Rather than going by the original, I made the character my own. [Beat] Gedächtnis: ...You think I'm just being lazy, yeah?
Art Evolution: Drossel and Gedächtnis have both received updated character designs in Charming — Gedächtnis in particular is significantly more detailed. These may simply be examples of older hardware, however, considering that Charming is a prequel to Fireball.
Badass Cape: Drossel wears one in an episode of Charming.
Bolivian Army Ending: Implied in the last episode of the first season. Conspicuously, the second season is a prequel.
Book Ends: The first episode of Charming starts with a dream Drossel has of standing in a classroom, in front of a chalkboard cluttered with writing. In the last episode of Charming, the classroom and chalboard is seen again as a representation of Drossel's memory and loss of which as she wipes the chalkboard clean. More importantly, however, is Drossel's wiping away "Gedächtnis", and the first season's Running Gag of Drossel's inability to remember Gedächtnis' name. When Drossel mentions that this might happen, Gedächtnis laughs. The fool.
In the beginning of Fireball, Drossel wants to see dolphins. At the end of Fireball Charming, Drossel asks Gedächtnis to count dolphins to help her fall asleep. (Since Charming is a prequel, the former is probably a consequence of the latter.)
Expressive Hair: In Charming, Drossel's "pigtails" have been replaced with a single articulated ponytail-like... thing.
It's kinda like a third leg attached to the back of her head. Creepy...
The Faceless: After Drossel has her Juno-XIII face taken off (to be refitted with the Juno-XIV), whatever was beneath is conspicuously not shown.
Flower Motifs: Blue Roses. Drossel gets one for her birthday. Apparently they are biohazardous. Also, the antiquated puppet representing human beings contain blue rose petals, and blue roses are components in Drossel's "bed"/stasis chamber.
Genre Savvy: At one point Drossel remembers having seen an anime. Gedächtnis also references anime in another episode. Though in this case, they were probably talking about Western cartoons.
Getting Crap Past the Radar: In episode 2 of Charming, Drossel makes an entrance by slowly sliding down metal poles and finishes it off with some stretching...
In episode 7 of Fireball, Gedächtnis mimicking Drossel says "Bad timing", which sounds very similar to "you bitch!" in Japanese.
The human-puppet throwing the Vulcan Salute ("live long and prosper") in the Charming finale. Maybe the Charming Mankind did show up for the Ball.
Drossel starts humming Daisy Bell as she is being prepared for stasis at the end of Charming.
On meeting a human in Charming ep 1, Drossel holds out her hand as if for a handshake, then uses the Curwen hand gestures from the movie Close Encounters of the Third Kind.
Drossel's eye beams from episode 1 are reminiscent of Astro Boy's eye beams, right down to being a gift from her father.
In the scene where Drossel plays baseball, Gedächtnis asks if her pitching form is Baritsu, which is the martial art practiced by Sherlock Holmes.
The mansion in the first season resembles the Tower of Babel seen in Metropolis, but the more detailed version of the mansion in Charming looks more like Genom Tower from Bubblegum Crisis.
Time Skip: Years and years pass between each episode. It doesn't affect much.
Title Drop: In the first season, "Fireball" refers to a plan devised by Windstille regarding the war with mankind. In the second season, "Charming" refers to the stage of mankind that will be devoid of senseless warmongering.
Widget Series: Maybe not as weird as some, but still quite... out there.