In the flashback to the first episode, Professor Oak says, "Here's your Pikachu!", which he doesn't say in the episode during that scene.
Misty tells Ash that real Pokémon trainers catch their Pokémon and he never caught any of his 6 Pokémon that they all just followed him. When actually Pikachu was his starter Pokémon and Butterfree was a lucky catch as a Caterpie. Ash actually battled and captured Pidgeotto and Bulbasaur using Pikachu. Only Charmander and Squirtle actually followed him.
Ash and Brock both address Pikachu as "Pika" when he discovers the lighthouse.