This is the second time someone, namely the giant Gengar and Alakazam, doesn't fall asleep to Jigglypuff's song, this time because Jigglypuff was too small to affect them.
The part about Eve writing a book at age nine was exclusive to the English version.
Alakazam uses a move similar to Psycho Cut, although the move was revealed three generations after.
The giant Gengar's ability to trap other people and Pokémon inside its body is somewhat similar to the fake Groudon from Pokémon: Jirachi - Wish Maker (2003). Team Rocket was also trapped inside both.
Ash says that the last Gengar he saw wasn't even as big as him, making reference to the events of Capture at the Pokémon Tower! (1997).