Beast, Cyclops, Jean Grey, Professor X and Storm do not appear. Gambit only appears as Mystique in a shape shift.
When Jubilee goes to answer the door the Beast mask is no longer there. Also after the door is opened, she has a bowl of candy but there was no way she could have picked one up, as there wasn't one around.
When Nightcrawler is talking to Jubilee at one point, you can see that he has 5 fingers instead of 3.
When Wolverine scares away the kids with the Beast mask, his arm is blue in the wide shot as they run away.
In the footage of the news report, a girl's skirt changes from purple to blue and back several times in the large crowd shot..
Wolverine: What's Mystique doin' here?
Rogue: She's Nightcrawler's mother, and Creed's mother, too!
Jubilee: Boy, talk about your soap operas!
Jubilee: oh nice. Beast won't appreciate you using his face to give them nightmares.
Wolverine: Isn't that what Halloween is all about?
Jubilee: Nooooo. It's about getting people to give you lots of candy and then trying to eat it all in one night.
Nightcrawler: Stop this! She is our mother! She gave you life!
Graydon Creed: She is a mutant! She gave me misery, and shame. She has to suffer...ALL MUTANTS MUST SUFFER!
Sabretooth: Well, well, well.
Creed: No! Sabretooth.
Sabretooth: If it isn't my son, the famous slayer of mutants. (picks up Creed)
Creed: No! No! Please!
Sabretooth: Now, son, come to Papa.
Nightcrawler is revealed to be 25 years old.
Though originally scripted between "Family Ties" and "The Lotus and The Steel", this episode ended up being made originally as the second "series finale", since it was produced as the 5th additional episode ordered following the show's initial 65. But high demand for the series led to a final 6 more shows ordered.
The three kids Trick or Treating are dressed like the Tarantula, Daredevil (his yellow original costume), and the Devil Dinosaur.
Mystique's age is unknown, but she has been around at least since the 1930's, when she would take the guise of a man named D. Raven.
Nightcrawler, Creed and Rogue are all related. How? Night and Creed are half brothers (different fathers), and Rogue was unofficially adopted by Mystique, their mother.