Ash catches a Caterpie which by the end of the episode evolves into a Metapod. The only time such an event occurs in the entirety of the series.
When Ash sees Pidgeotto, it is eating a worm and this is the first non-Pokémon animal to be seen in the show.
While Ash tells Caterpie "Counter", some have taken this to mean the move, which Caterpie cannot learn. In the original, Ash told it merely to "counter strike".
This is the first episode where Ash uses his signature catchphrase and pose when successfully catching a Pokémon. Each of Ash's traveling companions would attempt to copy this during the original series.
This is the first episode where type advantages are mentioned: Flying-type moves are effective against Bug-type Pokemon.