In the first generation of games Psychic Pokemon actually have the advantage over Ghost Pokemon.
Haunter leaves Ash to stay with Sabrina.
The Marsh Badge that Ash earns in this episode is his fourth Badge. However, in a rare deviation from the traditional sequence of Gyms, the Marsh Badge is officially the sixth Badge that the player earns in the games.
Ash earns the Marsh Badge due to Haunter making Sabrina and Kadabra laugh, thus making Kadabra unable to battle.
The book Night in the Haunted Tower is based on this and the two previous episodes.