During the scene when Negaduck is shooting at the poor defenseless bunny that Darkwing pretended to point out in the S.H.U.S.H. laboratory, Negaduck's mask switches from black to purple several times while he is shooting.
When this episode first aired, neither Stegmutt, Neptunia, or Gizmoduck have made appearances on the show despite that, as indicated by the dialogue, Darkwing has clearly had previous dealings with them.
In DuckTales, Gizmoduck was strong. But, in this episode he was not even able to hold up a small anvil.
Liquidator: (About Stegmutt) Just when you thought it was safe to commit crimes: A dinosaur!
Bushroot: Well I hope he's not a plant-eater!
Bushroot: Are you sure we're supposed to be here? This is a police station.
Liquidator: That's what the boss wants. And you know the boss. Kinda how and kinda wow.
Bushroot: I just hope we don't run into any policemen. (Walks right into a policeman) AAH!!!!!! Policeman.
(Bushroot and the policeman run away in opposite directions)
(After being knocked several miles across the city by Neptunia)
Darkwing: This is the third most painful moment of my life
Megavolt: How d'ya like your superhero?
Quackerjack: Fried: To a crackly crisp!
Darkwing:All set for our big night out Morganna? Lets get amorous.
Gosalyn:Darkwing Duck, crime fighter, bad Elvis impersonator.
Darkwing: This is the second most painful moment of my life.
Stegmutt: What was the first?
Darkwing: Put out the Darkwing, Put out the Darkwing.
Negaduck: (Stomping on Darkwing) This is for Megavolt, Bushroot, Liquidator, Quackerjack! And Negaduck makes five - the Fearsome Five!
Darkwing: Negaduck! So you're behind all this!
J. Gander: What-who-eh... two Darkwings?!
Darkwing/Negaduck: Don't you believe it, J. Gander? This fiend is my archnemesis, Negaduck!
Negaduck/Darkwing: Oh no, you don't! You're the liar known as Negaduck!
Darkwing/Negaduck: I beg to differ! You're the erroneous executor known only as Negaduck!"
Negaduck/Darkwing: OOOOH! Would you cut that out?!!
First appearance of Morgana Macawber (Darkwing's love interest).
First appearance of Negaduck.
First appearance of Gizmoduck.
First appearance of Neptunia.
First appearance of the Liquidator.
First appearence of Steggmutt.
This episode was releaed on Walt Disney home video titled: Justice Duck Unite!.
The second two-parter of the series, with the first being "Darkly Dawns The Duck."
This two-parter is available on home video.
Gizmoduck can be seen in DuckTales.
This is the first appearance of Negaduck (the current one).
Megavolt: Ah! I love the smell of voltage in the evening.
Direct reference to a famous quote from the movie "Apocalypse Now" by Colonel Kilgore (played by Robert Duvall) "I love the smell of napalm in the morning."
Gosalyn calls Stegmutt, "the uncanny Stegmutt!", a parody of the comic series Uncanny X-Men.
Justice League & Avengers
The Justice Ducks seem to have been inspired by both the Marvel Comics group The Avengers (Morgana=The Scarlet Witch, Stegmutt=The Incredible Hulk, Gizmoduck=Iron Man) and the DC Comics' Justice League (The name, Darkwing=Batman, Neptunia=Aquaman).
There is a group of DC Comics villains who call themselves the Fearsome Five.